Debug assertion in mariadb_backend.cc

If a connection has more than one queued up session command response that needs checking and the first one returns an answer that differs from the accepted one, the connection is closed but the response checking does not stop. This causes a debug assertion in MariaDBBackendConnection::track_query to be hit that can be called by MariaDBBackendConnection::track_response. The correct thing to do is to break out of the loop whenever a partial response or an error is received.