decimal.InvalidOperation exception is thrown with AWS Redshift.

* **asyncpg version**: 0.23.0
* **PostgreSQL version**: PostgreSQL 8.0.2 on i686-pc-linux-gnu, compiled by GCC gcc (GCC) 3.4.2 20041017 (Red Hat 3.4.2-6.fc3), Redshift 1.0.27314
* **Do you use a PostgreSQL SaaS? If so, which? Can you reproduce
the issue with a local PostgreSQL install?**: AWS Redshift.
* **Python version**: 3.6.13
* **Platform**: Linux 5.0.0-23-generic #24~18.04.1-Ubuntu SMP Mon Jul 29 16:12:28 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux
* **Do you use pgbouncer?**: no
* **Did you install asyncpg with pip?**: yes
* **If you built asyncpg locally, which version of Cython did you use?**: no
* **Can the issue be reproduced under both asyncio and
[uvloop](https://github.com/magicstack/uvloop)?**: no

I am trying to use asyncpg with AWS Redshift. I got an decimal.InvalidOperation exception when I try to run a simple query as follows. Is there any way to make asyncpg work with AWS Redshift? psycopg2 is working with AWS Redshift without major issue so I expect asyncpg could work with AWS Redshift also. Thank you in advance.

```
import asyncio
import asyncpg

async def run():
conn = await asyncpg.connect(
user='user', password='password', database='database', host='host')
values = await conn.fetch('SELECT 1.0 / 0.5')
await conn.close()

loop = asyncio.get_event_loop()
loop.run_until_complete(run())
```

```
$ python asyncpg_redshift.py
Traceback (most recent call last):
File "asyncpg_redshift.py", line 11, in
loop.run_until_complete(run())
File "/home/taeho/.pyenv/versions/3.6.13/lib/python3.6/asyncio/base_events.py", line 488, in run_until_complete
return future.result()
File "asyncpg_redshift.py", line 7, in run
values = await conn.fetch('SELECT 1.0 / 0.5')
File "/home/taeho/.pyenv/versions/3.6.13/envs/asyncpg/lib/python3.6/site-packages/asyncpg/connection.py", line 588, in fetch
record_class=record_class,
File "/home/taeho/.pyenv/versions/3.6.13/envs/asyncpg/lib/python3.6/site-packages/asyncpg/connection.py", line 1632, in _execute
ignore_custom_codec=ignore_custom_codec,
File "/home/taeho/.pyenv/versions/3.6.13/envs/asyncpg/lib/python3.6/site-packages/asyncpg/connection.py", line 1655, in __execute
ignore_custom_codec=ignore_custom_codec,
File "/home/taeho/.pyenv/versions/3.6.13/envs/asyncpg/lib/python3.6/site-packages/asyncpg/connection.py", line 1697, in _do_execute
result = await executor(stmt, None)
File "asyncpg/protocol/protocol.pyx", line 201, in bind_execute
File "asyncpg/protocol/coreproto.pyx", line 70, in asyncpg.protocol.protocol.CoreProtocol._read_server_messages
File "asyncpg/protocol/coreproto.pyx", line 212, in asyncpg.protocol.protocol.CoreProtocol._process__bind_execute
File "asyncpg/protocol/coreproto.pyx", line 497, in asyncpg.protocol.protocol.CoreProtocol._parse_data_msgs
File "asyncpg/protocol/protocol.pyx", line 781, in asyncpg.protocol.protocol.BaseProtocol._decode_row
File "asyncpg/protocol/prepared_stmt.pyx", line 295, in asyncpg.protocol.protocol.PreparedStatementState._decode_row
File "asyncpg/protocol/codecs/base.pyx", line 289, in asyncpg.protocol.protocol.Codec.decode
File "asyncpg/protocol/codecs/base.pyx", line 196, in asyncpg.protocol.protocol.Codec.decode_scalar
File "asyncpg/pgproto/./codecs/numeric.pyx", line 287, in asyncpg.pgproto.pgproto.numeric_decode_binary
File "asyncpg/pgproto/./codecs/numeric.pyx", line 279, in asyncpg.pgproto.pgproto.numeric_decode_binary_ex
decimal.InvalidOperation: []
$
```

I think the strange result of the following query could be also related.

```
import asyncio
import asyncpg

async def run():
conn = await asyncpg.connect(
user='user', password='password', database='database', host='host')
values = await conn.fetch('SELECT CAST(1.0 AS DECIMAL(10, 1))')
print(values)
await conn.close()

loop = asyncio.get_event_loop()
loop.run_until_complete(run())
```

```
$ python asyncpg_redshift2.py
[]
$
```

Start by running the two provided Python queries against AWS Redshift and compare their results with the traceback. Inspect asyncpg/pgproto/./codecs/numeric.pyx, where the failure occurs during numeric decoding, and determine how Redshift’s returned numeric data differs from PostgreSQL’s expected representation. Done means the incompatibility is reproduced and a supported behavior or narrowly scoped fix is established.

Written by the indexing model from the issue text.