Deliver hangups only to valid DCBs

If a DCB was closed and a hangup event was sent to it via
dcb_hangup_foreach shortly after it was closed, the DCB would still
receive it even if it was closed. To prevent this, events must only be
delivered to DCBs if they haven't been closed.

server/modules/protocol/MySQL/mariadbbackend/mysql_backend.cc

@@ -1354,6 +1354,7 @@ static int gw_error_backend_event(DCB* dcb)
  */
 static int gw_backend_hangup(DCB* dcb)
 {
+    mxb_assert(dcb->n_close == 0);
     MXS_SESSION* session = dcb->session;
 
     if (dcb->persistentstart)