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/util.c

@@ -992,6 +992,7 @@ meta_get_locale_direction (void)
       return META_LOCALE_DIRECTION_RTL;
     default:
       g_assert_not_reached ();
+      return 0;
     }
 }