lib-smtp: smtp-server-cmd-auth - Fix AUTH response error handling so that it stops...

lib-smtp: smtp-server-cmd-auth - Fix AUTH response error handling so that it stops reading more input.

Otherwise, it may try to read more data from the stream as the next AUTH
response, which causes an assert crash in the command parser later on. Even when
the parser finds no input from the stream, it will advance its state
towards AUTH response parsing, which is a problem when the next command is
subsequently being parsed.

Panic was:

Panic: file smtp-command-parser.c: line 438 (smtp_command_parse_next): assertion failed: (!parser->auth_response || parser->state.state == SMTP_COMMAND_PARSE_STATE_INIT || parser->state.state == SMTP_COMMAND_PARSE_STATE_ERROR)