Pass the right worker into isc__nm_async_sockstop()

The intention behind 'isc__nmsocket_stop()' was that the function
sends notifications on every worker thread, making them synchronise on
the barrier, then the initiating thread waits on it, too. This way we
ensure than no other operation will start when we shutting down the
listener.

However, it seems that due to mistake we have been passing the wrong
worker pointer into isc__nm_async_sockstop() from within the context
of an worker thread which has initiated shutting down. While
effectively we have not been using the pointer in this case, it could
cause maintenance issues later. This commit fixes that.

lib/isc/netmgr/netmgr.c

@@ -2756,7 +2756,7 @@ isc__nmsocket_stop(isc_nmsocket_t *listener) {
 	}
 
 	if (isc__nm_in_netthread()) {
-		isc__nm_async_sockstop(&listener->mgr->workers[0],
+		isc__nm_async_sockstop(&listener->mgr->workers[isc_nm_tid()],
 				       (isc__netievent_t *)&ievent);
 	}
 }