Don't write errors to dummy sessions

If a DCB is closed before a response to the handshake packet is received,
the DCB's session will point to the dummy session. In this case no error
should be written to the DCB.

server/modules/protocol/MySQL/mariadbclient/mysql_client.cc

@@ -1416,7 +1416,7 @@ static int gw_client_hangup_event(DCB *dcb)
         goto retblock;
     }
 
-    if (!session_valid_for_pool(session))
+    if (session->state != SESSION_STATE_DUMMY && !session_valid_for_pool(session))
     {
         // The client did not send a COM_QUIT packet
         modutil_send_mysql_err_packet(dcb, 0, 0, 1927, "08S01", "Connection killed by MaxScale");