MXS-3055: Prevent sending of an extra COM_QUIT

If the protocol routes a COM_QUIT packet to the backend, it must not
generate a packet when it is shutting down. This could cause unexpected
write errors if the backend server managed to close the socket before the
write was done.

server/modules/protocol/MySQL/mysql_common.cc

@@ -64,6 +64,7 @@ MySQLProtocol* mysql_protocol_init(DCB* dcb, int fd)
     p->num_eof_packets = 0;
     p->large_query = false;
     p->track_state = false;
+    p->send_com_quit = true;
     /*< Assign fd with protocol */
     p->fd = fd;
     p->owner_dcb = dcb;