Schema reader crashes when replaying protobuf

# What happened?

Adding a protobuf schema causes an exception and schema registry becomes unresponsive.

# What did you expect to happen?

Schema to be added and replay to continue. API to continue to be available.

# What else do we need to know?

Tried with `3.14.0` and `3.15.0`

Schema appears to be added OK, but replay fails in the same way, restarting the container results in the below error:

Starting Karapace Schema Registry
Cannot enable Sentry.io sending: importing 'sentry_sdk' failed
karapace MainThread INFO Karapace initialized
root MainThread INFO

Starting karapace schema registry

karapace.instrumentation.prometheus MainThread INFO Setting up prometheus metrics
karapace.coordinator.schema_coordinator MainThread INFO Metadata for topic has changed from [] to [Assignment(member_id='v0', metadata=b'{"version":2,"karapace_version":"3.15.0","host":"karapace-schema-registry","port":8081,"scheme":"http","master_eligibility":true}')].
karapace.coordinator.schema_coordinator MainThread INFO Discovered coordinator 0 for group karapace-schema-registry
karapace.coordinator.schema_coordinator MainThread INFO (Re-)joining group karapace-schema-registry
karapace.coordinator.schema_coordinator MainThread INFO Joined group 'karapace-schema-registry' (generation 16) with member_id karapace-schema-registry-73c87c16-78ce-4997-b000-456cd1111889
karapace.coordinator.schema_coordinator MainThread INFO Joined as follower.
karapace.coordinator.schema_coordinator MainThread INFO Successfully synced group karapace-schema-registry with generation 16
karapace.coordinator.schema_coordinator MainThread INFO Join complete, generation 16, member_id: 'karapace-schema-registry-73c87c16-78ce-4997-b000-456cd1111889', protocol: 'v0', member_assignment_bytes: b'{"master":"karapace-schema-registry-ad1354cc-1653-4808-8b57-6aeb6b1ec925","master_identity":{"version":2,"karapace_version":"3.14.0","host":"karapace-schema-registry","port":8081,"scheme":"http","master_eligibility":true},"error":0}'
karapace.kafka.consumer.KafkaConsumer schema-reader INFO Subscribing to topics ['_schemas'] and patterns []
karapace.schema_reader schema-reader INFO [Schema Topic] Creating '_schemas'
karapace.kafka.admin.KafkaAdminClient schema-reader INFO Creating new topic NewTopic(topic=_schemas,num_partitions=1) with replication factor 1
karapace.schema_reader schema-reader WARNING [Schema Topic] Already exists '_schemas'
aiohttp.access MainThread INFO 0.000669s - "GET /_health HTTP/1.1" 200 "kube-probe/1.26" response=570b request_body=-b
aiohttp.access MainThread INFO 0.001064s - "GET /_health HTTP/1.1" 200 "kube-probe/1.26" response=570b request_body=-b
karapace.schema_reader schema-reader INFO Replay progress (3.27): -1/0 (0 %) (recs/s -0.30561052807405376)
karapace.schema_reader schema-reader INFO Replay progress (0.2): -1/0 (0 %) (recs/s 0.0)
karapace.schema_reader schema-reader INFO Replay progress (0.2): -1/0 (0 %) (recs/s 0.0)
karapace.schema_reader schema-reader INFO Replay progress (0.2): -1/0 (0 %) (recs/s 0.0)
karapace.schema_reader schema-reader INFO Replay progress (0.2): -1/0 (0 %) (recs/s 0.0)
karapace.schema_reader schema-reader INFO Replay progress (0.2): -1/0 (0 %) (recs/s 0.0)
karapace.schema_reader schema-reader INFO Replay progress (0.2): -1/0 (0 %) (recs/s 0.0)
karapace.schema_reader schema-reader INFO Replay progress (0.2): -1/0 (0 %) (recs/s 0.0)
karapace.schema_reader schema-reader INFO Replay progress (0.2): -1/0 (0 %) (recs/s 0.0)
karapace.schema_reader schema-reader INFO Replay progress (0.2): -1/0 (0 %) (recs/s 0.0)
karapace.schema_reader schema-reader INFO Replay progress (0.2): -1/0 (0 %) (recs/s 0.0)
karapace.schema_reader schema-reader INFO Replay progress (0.2): -1/0 (0 %) (recs/s 0.0)
karapace.schema_reader schema-reader INFO Replay progress (0.2): -1/0 (0 %) (recs/s 0.0)
karapace.coordinator.schema_coordinator MainThread WARNING Heartbeat failed for group karapace-schema-registry because it is rebalancing
karapace.coordinator.schema_coordinator MainThread INFO (Re-)joining group karapace-schema-registry
karapace.coordinator.schema_coordinator MainThread INFO Joined group 'karapace-schema-registry' (generation 17) with member_id karapace-schema-registry-73c87c16-78ce-4997-b000-456cd1111889
karapace.coordinator.schema_coordinator MainThread INFO Elected group leader -- performing partition assignments using v0
karapace.coordinator.schema_coordinator MainThread INFO Creating assignment: 'karapace-schema-registry-73c87c16-78ce-4997-b000-456cd1111889', protocol: 'v0', members: [JoinGroupMemberData(member_id='karapace-schema-registry-73c87c16-78ce-4997-b000-456cd1111889', member_data=b'{"version":2,"karapace_version":"3.15.0","host":"karapace-schema-registry","port":8081,"scheme":"http","master_eligibility":true}')]
karapace.coordinator.schema_coordinator MainThread INFO Chose: 'karapace-schema-registry-73c87c16-78ce-4997-b000-456cd1111889' with url: 'http://karapace-schema-registry:8081' as the master
karapace.coordinator.schema_coordinator MainThread INFO Successfully synced group karapace-schema-registry with generation 17
karapace.coordinator.schema_coordinator MainThread INFO Join complete, generation 17, member_id: 'karapace-schema-registry-73c87c16-78ce-4997-b000-456cd1111889', protocol: 'v0', member_assignment_bytes: b'{"master":"karapace-schema-registry-73c87c16-78ce-4997-b000-456cd1111889","master_identity":{"version":2,"karapace_version":"3.15.0","host":"karapace-schema-registry","port":8081,"scheme":"http","master_eligibility":true},"error":0}'
karapace.schema_reader schema-reader INFO Replay progress (0.2): -1/0 (0 %) (recs/s 0.0)
karapace.schema_reader schema-reader INFO Replay progress (0.2): -1/0 (0 %) (recs/s 0.0)
karapace.schema_reader schema-reader INFO Replay progress (0.23): -1/0 (0 %) (recs/s 0.0)
karapace.schema_reader schema-reader ERROR Unexpected exception in schema reader loop
Traceback (most recent call last):
File "/venv/lib/python3.10/site-packages/karapace/protobuf/schema.py", line 267, in __init__
self.proto_file_element = deserialize(schema)
File "/venv/lib/python3.10/site-packages/karapace/protobuf/serialization.py", line 157, in deserialize
serialized_pb = base64.b64decode(schema_b64, validate=True)
File "/usr/local/lib/python3.10/base64.py", line 86, in b64decode
raise binascii.Error('Non-base64 digit found')
binascii.Error: Non-base64 digit found

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/venv/lib/python3.10/site-packages/karapace/schema_reader.py", line 244, in run
self.handle_messages()
File "/venv/lib/python3.10/site-packages/karapace/schema_reader.py", line 390, in handle_messages
self.handle_msg(key, value)
File "/venv/lib/python3.10/site-packages/karapace/schema_reader.py", line 586, in handle_msg
self._handle_msg_schema(key, value)
File "/venv/lib/python3.10/site-packages/karapace/schema_reader.py", line 539, in _handle_msg_schema
parsed_schema = parse_protobuf_schema_definition(
File "/venv/lib/python3.10/site-packages/karapace/schema_models.py", line 75, in parse_protobuf_schema_definition
ProtobufSchema(schema_definition, references, dependencies)
File "/venv/lib/python3.10/site-packages/karapace/protobuf/schema.py", line 269, in __init__
self.proto_file_element = ProtoParser.parse(DEFAULT_LOCATION, schema)
File "/venv/lib/python3.10/site-packages/karapace/protobuf/proto_parser.py", line 629, in parse
return proto_parser.read_proto_file()
File "/venv/lib/python3.10/site-packages/karapace/protobuf/proto_parser.py", line 104, in read_proto_file
declaration = self.read_declaration(documentation, Context.FILE)
File "/venv/lib/python3.10/site-packages/karapace/protobuf/proto_parser.py", line 205, in read_declaration
result = self.read_message(location, documentation)
File "/venv/lib/python3.10/site-packages/karapace/protobuf/proto_parser.py", line 245, in read_message
declared = self.read_declaration(nested_documentation, Context.MESSAGE)
File "/venv/lib/python3.10/site-packages/karapace/protobuf/proto_parser.py", line 215, in read_declaration
result = self.read_one_of(documentation)
File "/venv/lib/python3.10/site-packages/karapace/protobuf/proto_parser.py", line 453, in read_one_of
fields.append(self.read_field_with_label(location, nested_documentation, None, atype))
File "/venv/lib/python3.10/site-packages/karapace/protobuf/proto_parser.py", line 389, in read_field_with_label
self.reader.require("=")
File "/venv/lib/python3.10/site-packages/karapace/protobuf/syntax_reader.py", line 35, in require
self.expect(self.read_char() == c, f"expected '{c}'")
File "/venv/lib/python3.10/site-packages/karapace/protobuf/syntax_reader.py", line 361, in expect
self.unexpected(message, location)
File "/venv/lib/python3.10/site-packages/karapace/protobuf/syntax_reader.py", line 370, in unexpected
raise IllegalStateException(f"Syntax error in {str(location)}: {message}")
karapace.protobuf.exception.IllegalStateException: Syntax error in :14:54: expected '='
karapace.schema_reader schema-reader INFO Replay progress (0.25): -1/0 (0 %) (recs/s 0.0)
karapace.schema_reader schema-reader INFO Replay progress (0.21): -1/0 (0 %) (recs/s 0.0)
karapace.schema_reader schema-reader INFO Replay progress (0.22): -1/0 (0 %) (recs/s 0.0)
karapace.schema_reader schema-reader INFO Replay progress (0.21): -1/0 (0 %) (recs/s 0.0)
karapace.schema_reader schema-reader INFO Replay progress (0.21): -1/0 (0 %) (recs/s 0.0)
karapace.schema_reader schema-reader INFO Replay progress (0.2): -1/0 (0 %) (recs/s 0.0)
karapace.schema_reader schema-reader INFO Replay progress (0.21): -1/0 (0 %) (recs/s 0.0)
karapace.schema_reader schema-reader INFO Replay progress (0.21): -1/0 (0 %) (recs/s 0.0)
karapace.schema_reader schema-reader INFO Replay progress (0.22): -1/0 (0 %) (recs/s 0.0)
karapace.schema_reader schema-reader INFO Replay progress (0.21): -1/0 (0 %) (recs/s 0.0)
karapace.schema_reader schema-reader INFO Replay progress (0.2): -1/0 (0 %) (recs/s 0.0)
karapace.schema_reader schema-reader INFO Replay progress (0.2): -1/0 (0 %) (recs/s 0.0)
karapace.schema_reader schema-reader INFO Replay progress (0.2): -1/0 (0 %) (recs/s 0.0)
karapace.schema_reader schema-reader INFO Replay progress (0.21): -1/0 (0 %) (recs/s 0.0)
karapace.schema_reader schema-reader INFO Replay progress (0.21): -1/0 (0 %) (recs/s 0.0)
karapace.schema_reader schema-reader INFO Replay progress (0.21): -1/0 (0 %) (recs/s 0.0)
karapace.schema_reader schema-reader INFO Replay progress (0.21): -1/0 (0 %) (recs/s 0.0)
karapace.schema_reader schema-reader INFO Replay progress (0.21): -1/0 (0 %) (recs/s 0.0)
karapace.schema_reader schema-reader INFO Replay progress (0.21): -1/0 (0 %) (recs/s 0.0)
karapace.schema_reader schema-reader INFO Replay progress (0.21): -1/0 (0 %) (recs/s 0.0)
karapace.schema_reader schema-reader INFO Replay progress (0.2): -1/0 (0 %) (recs/s 0.0)
karapace.schema_reader schema-reader INFO Replay progress (0.23): -1/0 (0 %) (recs/s 0.0)
karapace.schema_reader schema-reader INFO Replay progress (0.21): -1/0 (0 %) (recs/s 0.0)
karapace.schema_reader schema-reader INFO Replay progress (0.2): -1/0 (0 %) (recs/s 0.0)
karapace.schema_reader schema-reader INFO Replay progress (0.22): -1/0 (0 %) (recs/s 0.0)
karapace.schema_reader schema-reader INFO Replay progress (0.25): -1/0 (0 %) (recs/s 0.0)
karapace.schema_reader schema-reader INFO Replay progress (0.21): -1/0 (0 %) (recs/s 0.0)
karapace.schema_reader schema-reader INFO Replay progress (0.2): -1/0 (0 %) (recs/s 0.0)
karapace.schema_reader schema-reader INFO Replay progress (0.2): -1/0 (0 %) (recs/s 0.0)
karapace.schema_reader schema-reader INFO Replay progress (0.21): -1/0 (0 %) (recs/s 0.0)
karapace.schema_reader schema-reader INFO Replay progress (0.21): -1/0 (0 %) (recs/s 0.0)

This log seems to continue repeating forever, with various similar values in replay progress.