[PATCH] Fix unbounded authentication exchanges during PQconnectPoll()

Hello,

This is closely related to the prior conversation at [1]. There are a
couple places in CONNECTION_AWAITING_RESPONSE where libpq will read a
huge number of bytes from a server that we really should have hung up on.

The attached patch adds a length check for the v2 error compatibility
case, and updates the v3 error handling to jump to error_return rather
than asking for more data. The existing error_return paths have been
updated for consistency.

Thanks,
--Jacob

[1]
https://www.postgresql.org/message-id/a5c5783d-73f3-acbc-997f-1649a7406029%40timescale.com