Fix builds with G_DISABLE_ASSERT

Commit 25f416c13d added additional compilation warnings, including
-Werror=return-type. There are several places where this results
in build failures if `g_assert_not_reached()` is disabled at compile
time and the compiler misses a return value.

https://gitlab.gnome.org/GNOME/mutter/issues/447

src/core/window.c

@@ -5498,6 +5498,7 @@ meta_window_get_workspaces (MetaWindow *window)
     return NULL;
   else
     g_assert_not_reached ();
+  return NULL;
 }
 
 static void