ui/vnc: Fix qemu abort when query vnc info

When there is no display device on qemu machine,
and user only access qemu by remote vnc.
At the same time user input `info vnc` by QMP,
the qemu will abort.

To avoid the abort above, I add display device check,
when query vnc info in qmp_query_vnc_servers().

Reviewed-by: Marc-AndréLureau <marcandre.lureau@redhat.com>
Signed-off-by: Alano Song <AlanoSong@163.com>
[ Marc-André - removed useless Error *err ]
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-ID: <20251125131955.7024-1-AlanoSong@163.com>

ui/vnc.c

@@ -556,9 +556,12 @@ VncInfo2List *qmp_query_vnc_servers(Error **errp)
         qmp_query_auth(vd->auth, vd->subauth, &info->auth,
                        &info->vencrypt, &info->has_vencrypt);
         if (vd->dcl.con) {
-            dev = DEVICE(object_property_get_link(OBJECT(vd->dcl.con),
-                                                  "device", &error_abort));
-            info->display = g_strdup(dev->id);
+            Object *obj = object_property_get_link(OBJECT(vd->dcl.con),
+                                                   "device", NULL);
+            if (obj) {
+                dev = DEVICE(obj);
+                info->display = g_strdup(dev->id);
+            }
         }
         if (vd->listener != NULL) {
             nsioc = qio_net_listener_nsioc(vd->listener);