[rpc] RPC decoder allocates before validating field length

[fresh-borzoni]

Search before asking

• I searched in the issues and found nothing similar.

Description

The generated protobuf field decoders read a length-delimited field's length prefix and allocate for it before checking that many bytes are actually present.

So a small request with a large length prefix makes the server allocate a big array on the Netty event-loop
thread before it reads.

Willingness to contribute

• I'm willing to submit a PR!