SIGSEGV on startup with Confluent Kafka

Nobody has claimed this yet.

Hi there,

Burrow panics at startup with Confluent Kafka 3.2.2 (implementing Apache Kafka 0.10.2.1).
I'm running the latest version of Burrow which includes fix from #345 .

burrow.out :
```
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x30 pc=0x859ffa]

goroutine 127 [running]:
github.com/linkedin/Burrow/core/internal/consumer.(*KafkaClient).processConsumerOffsetsMessage(0xc42023c090, 0x0)
$GOPATH/src/github.com/linkedin/Burrow/core/internal/consumer/kafka_client.go:233 +0x3a
github.com/linkedin/Burrow/core/internal/consumer.(*KafkaClient).partitionConsumer(0xc42023c090, 0xcbd2c0, 0xc420aa8380)
$GOPATH/src/github.com/linkedin/Burrow/core/internal/consumer/kafka_client.go:170 +0x562
created by github.com/linkedin/Burrow/core/internal/consumer.(*KafkaClient).startKafkaConsumer
$GOPATH/src/github.com/linkedin/Burrow/core/internal/consumer/kafka_client.go:225 +0x5af
```

burrow.log :
```
{"level":"info","ts":1518192593.3013716,"msg":"Started Burrow"}
{"level":"info","ts":1518192593.3016908,"msg":"configuring","type":"coordinator","name":"zookeeper"}
{"level":"info","ts":1518192593.3028903,"msg":"configuring","type":"coordinator","name":"storage"}
{"level":"info","ts":1518192593.3029835,"msg":"configuring","type":"module","coordinator":"storage","class":"inmemory","name":"default"}
{"level":"info","ts":1518192593.3031235,"msg":"configuring","type":"coordinator","name":"evaluator"}
{"level":"info","ts":1518192593.3031826,"msg":"configuring","type":"module","coordinator":"evaluator","class":"caching","name":"default"}
{"level":"info","ts":1518192593.3032408,"msg":"configuring","type":"coordinator","name":"httpserver"}
{"level":"info","ts":1518192593.3033948,"msg":"configuring","type":"coordinator","name":"notifier"}
{"level":"info","ts":1518192593.3034384,"msg":"configuring","type":"coordinator","name":"cluster"}
{"level":"info","ts":1518192593.3034916,"msg":"configuring","type":"module","coordinator":"cluster","class":"kafka","name":"local"}
{"level":"info","ts":1518192593.3040285,"msg":"configuring","type":"coordinator","name":"consumer"}
{"level":"info","ts":1518192593.3041415,"msg":"configuring","type":"module","coordinator":"consumer","class":"kafka_zk","name":"local_zk"}
{"level":"info","ts":1518192593.304852,"msg":"configuring","type":"module","coordinator":"consumer","class":"kafka","name":"local"}
{"level":"info","ts":1518192593.3053997,"msg":"starting","type":"coordinator","name":"zookeeper"}
{"level":"info","ts":1518192593.3076718,"msg":"Connected to 127.0.0.1:2181","type":"coordinator","name":"zookeeper"}
{"level":"info","ts":1518192593.3122435,"msg":"Authenticated: id=99496255347884036, timeout=6000","type":"coordinator","name":"zookeeper"}
{"level":"info","ts":1518192593.312441,"msg":"Re-submitting `0` credentials after reconnect","type":"coordinator","name":"zookeeper"}
{"level":"info","ts":1518192593.3162775,"msg":"starting","type":"coordinator","name":"storage"}
{"level":"info","ts":1518192593.3163555,"msg":"starting","type":"module","coordinator":"storage","class":"inmemory","name":"default"}
{"level":"info","ts":1518192593.316457,"msg":"starting","type":"coordinator","name":"evaluator"}
{"level":"info","ts":1518192593.3164802,"msg":"starting","type":"module","coordinator":"evaluator","class":"caching","name":"default"}
{"level":"info","ts":1518192593.3164997,"msg":"starting","type":"coordinator","name":"httpserver"}
{"level":"info","ts":1518192593.316799,"msg":"started listener","type":"coordinator","name":"httpserver","listener":"[::]:8000"}
{"level":"info","ts":1518192593.3168387,"msg":"starting","type":"coordinator","name":"notifier"}
{"level":"info","ts":1518192593.3168676,"msg":"starting","type":"coordinator","name":"cluster"}
{"level":"info","ts":1518192593.3168843,"msg":"starting","type":"module","coordinator":"cluster","class":"kafka","name":"local"}
{"level":"info","ts":1518192593.3624427,"msg":"starting","type":"coordinator","name":"consumer"}
{"level":"info","ts":1518192593.3625522,"msg":"starting","type":"module","coordinator":"consumer","class":"kafka_zk","name":"local_zk"}
{"level":"info","ts":1518192593.364876,"msg":"Failed to connect to [::1]:2181: dial tcp [::1]:2181: connect: network is unreachable","type":"module","coordinator":"consumer","class":"kafka_zk","name":"local_zk"}
{"level":"info","ts":1518192593.3651643,"msg":"Connected to 127.0.0.1:2181","type":"module","coordinator":"consumer","class":"kafka_zk","name":"local_zk"}
{"level":"info","ts":1518192593.3692389,"msg":"Authenticated: id=99496255347884037, timeout=30000","type":"module","coordinator":"consumer","class":"kafka_zk","name":"local_zk"}
{"level":"info","ts":1518192593.369343,"msg":"Re-submitting `0` credentials after reconnect","type":"module","coordinator":"consumer","class":"kafka_zk","name":"local_zk"}
{"level":"info","ts":1518192593.374514,"msg":"starting","type":"module","coordinator":"consumer","class":"kafka","name":"local"}
{"level":"info","ts":1518192593.3837266,"msg":"starting consumers","type":"module","coordinator":"consumer","class":"kafka","name":"local","topic":"__consumer_offsets","count":50}
{"level":"info","ts":1518192593.4269505,"msg":"starting evaluations","type":"coordinator","name":"notifier"}
```

burrow.toml :
```
[general]
pidfile="burrow.pid"
stdout-logfile="burrow.out"
access-control-allow-origin="mysite.example.com"

[logging]
filename="logs/burrow.log"
level="info"
maxsize=100
maxbackups=30
maxage=10
use-localtime=false
use-compression=true

[zookeeper]
servers=[ "localhost:2181" ]
timeout=6
root-path="/burrow"

[client-profile.test]
client-id="burrow-test"
kafka-version="0.10.2.1"

[cluster.local]
class-name="kafka"
servers=[ "localhost:9092" ]
client-profile="test"
topic-refresh=60
offset-refresh=15

[consumer.local]
class-name="kafka"
cluster="local"
servers=[ "localhost:9092" ]
client-profile="test"
group-whitelist=""
start-latest=false

[consumer.local_zk]
class-name="kafka_zk"
cluster="local"
servers=["localhost:2181" ]
zookeeper-path=""
zookeeper-timeout=30
group-whitelist=""

[httpserver.default]
address=":8000"

[storage.default]
class-name="inmemory"
workers=5
intervals=10
expire-group=604800
min-distance=0
```
I had done some tests with the same configuration and the latest Apache Kafka 1.0.0 on the same machine, and Burrow had run smoothly.

Start in core/internal/consumer/kafka_client.go at the reported stack frames on lines 233, 170, and 225. Reproduce startup with Confluent Kafka 3.2.2, Apache Kafka 0.10.2.1, and the supplied burrow.toml, then compare with the reported Kafka 1.0.0 setup. Done means Burrow starts without the nil-pointer panic while consuming __consumer_offsets.

Written by the indexing model from the issue text.