wayland/outputs: Fix potential crash when output has no monitor

bind_output() creates output interface resource, but does not
set implementation for it when wayland_output->monitor is NULL.
However, when the wayland library is running wl_closure_invoke(),
it expects the implementation to be non-NULL, and if not, it just
segfaults mutter by NULL pointer dereference.

This commit tries to address this issue by setting an implementation
when wayland_output->monitor is NULL. This could help prevent crash
when resuming from suspend or hotplugging displays.

Fixes: https://gitlab.gnome.org/GNOME/mutter/-/issues/2570
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2827>

src/wayland/meta-wayland-outputs.c

@@ -317,7 +317,12 @@ bind_output (struct wl_client *client,
 
   monitor = wayland_output->monitor;
   if (!monitor)
-    return;
+    {
+      wl_resource_set_implementation (resource,
+                                      &meta_wl_output_interface,
+                                      NULL, NULL);
+      return;
+    }
 
   wayland_output->resources = g_list_prepend (wayland_output->resources,
                                               resource);