gdir: shutup a warning when building with msvc

This way we avoid the warning:
warning C4716: 'g_dir_new_from_dirp': must return a value

glib/gdir.c

@@ -189,6 +189,8 @@ g_dir_new_from_dirp (gpointer dirp)
   return dir;
 #else
   g_assert_not_reached ();
+
+  return NULL;
 #endif
 }