Don't send hangup error on connection close

If the client has sent a COM_QUIT, the hangup error message should not be
written.

server/modules/protocol/MySQL/MySQLClient/mysql_client.c

@@ -1446,7 +1446,11 @@ static int gw_client_hangup_event(DCB *dcb)
         goto retblock;
     }
 
-    modutil_send_mysql_err_packet(dcb, 0, 0, 1927, "08S01", "Connection killed by MaxScale");
+    if (!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");
+    }
     dcb_close(dcb);
 
 retblock: