Handle a BAD response in AUTH PLAIN w/o initial response (#1237)

Handle a BAD response in AUTH PLAIN w/o initial response (#1237)

The incorrect assumption was that the first line of an AUTH PLAIN w/o
I-R was always successful, so the client could write the second line and
end up with a readable socket that would be activated by the server
sending the final OK or BAD response.
If however the first line results in a BAD response already, the server
wouldn't write anything else and the client would be left polling for an
additional response that never arrives, effectively hanging the process.

Issue #1236