Forbid SLAVE STOP if the thread executing the query has locked

    start_slave(thd);

    break;

  case SQLCOM_SLAVE_STOP:

   /*

      if the client thread has locked tables, a deadlock is possible.

      Assume that

      - the client thread does LOCK TABLE t READ.

      - then the master updates t.

      - then the SQL slave thread wants to update t,

      so it waits for the client thread because t is locked by it.

      - then the client thread does SLAVE STOP.

      SLAVE STOP waits for the SQL slave thread to terminate its

      update t, which waits for the client thread because t is locked by it.

      To prevent that, refuse SLAVE STOP if the

      client thread has locked tables

    */

    if (thd->locked_tables || thd->active_transaction())

    {

      send_error(&thd->net,ER_LOCK_OR_ACTIVE_TRANSACTION);

      break;

    }

    stop_slave(thd);

    break;