[mutter] main: Avoid calling meta_wayland_compositor_get_default() on X11

[Marge Bot <marge-bot src gnome org>]

commit ea626a50594513faa86174ae7dd58101d7c8dd2d
Author: Daniel van Vugt <daniel van vugt canonical com>
Date:   Wed Jun 2 16:45:04 2021 +0800

    main: Avoid calling meta_wayland_compositor_get_default() on X11
    
    Because it contains an assertion that will fail, and crash.
    Started in 301d2c55c6.
    
    Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1883>

 src/core/main.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)
---
diff --git a/src/core/main.c b/src/core/main.c
index b7b5d14490..3d82773ba6 100644
--- a/src/core/main.c
+++ b/src/core/main.c
@@ -309,13 +309,16 @@ meta_finalize (void)
   MetaDisplay *display = meta_get_display ();
   MetaBackend *backend = meta_get_backend ();
 #ifdef HAVE_WAYLAND
-  MetaWaylandCompositor *compositor = meta_wayland_compositor_get_default ();
+  MetaWaylandCompositor *compositor = NULL;
 #endif
 
   if (backend)
     meta_backend_prepare_shutdown (backend);
 
 #ifdef HAVE_WAYLAND
+  if (meta_is_wayland_compositor ())
+    compositor = meta_wayland_compositor_get_default ();
+
   if (compositor)
     meta_wayland_compositor_prepare_shutdown (compositor);
 #endif