wayland: No xdg-output events without a logical monitor

To avoid a known race condition in the wl_output protocol documented in
https://phabricator.freedesktop.org/T7722, mutter delays the `wl_output`
destruction but nullify the `logical_monitor` associated with the
`wl_output` and the binding routine `bind_output()` makes sure not to
send wl_output events if the `logical_monitor` is `NULL` (see commit
1923db97).

The binding routine for `xdg_output` however does not check for such a
condition, hence if the output configuration changes while a client is
binding to xdg-output (typically Xwayland at startup), mutter would
crash while trying to access the `logical_monitor` which was nullified
by the change in configuration.

Just like `bind_output()` does for wl_output, do not send xdg-output
events if there is no `logical_monitor` yet.

Closes: https://gitlab.gnome.org/GNOME/mutter/issues/194

src/wayland/meta-wayland-outputs.c

@@ -591,6 +591,9 @@ meta_xdg_output_manager_get_xdg_output (struct wl_client   *client,
   wayland_output->xdg_output_resources =
     g_list_prepend (wayland_output->xdg_output_resources, xdg_output_resource);
 
+  if (!wayland_output->logical_monitor)
+    return;
+
   send_xdg_output_events (xdg_output_resource,
                           wayland_output,
                           wayland_output->logical_monitor,