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/wayland/meta-wayland-buffer.c

@@ -373,6 +373,7 @@ meta_wayland_buffer_attach (MetaWaylandBuffer *buffer,
     }
 
   g_assert_not_reached ();
+  return FALSE;
 }
 
 CoglTexture *