Examples of IPROTO decoding issues: #3900, #1928, #6781.
Patch adds a number of fuzzing tests that covers IPROTO decoding:

- xrow_decode_auth
- xrow_decode_begin
- xrow_decode_call
- xrow_decode_dml
- xrow_decode_id
- xrow_decode_raft
- xrow_decode_sql
- xrow_decode_watch
- xrow_greeting_decode

NO_DOC=testing
NO_CHANGELOG=testing

(cherry picked from commit 46cacf35)