onscreen/native: Replace an assertion that double buffering is the maximum

Because it soon won't be the maximum. But we do want to verify that the
frame info queue is not empty, to avoid NULL dereferencing and catch logic
errors.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1441>

src/backends/native/meta-onscreen-native.c

@@ -199,7 +199,7 @@ meta_onscreen_native_notify_frame_complete (CoglOnscreen *onscreen)
 
   info = cogl_onscreen_pop_head_frame_info (onscreen);
 
-  g_assert (!cogl_onscreen_peek_head_frame_info (onscreen));
+  g_return_if_fail (info);
 
   _cogl_onscreen_notify_frame_sync (onscreen, info);
   _cogl_onscreen_notify_complete (onscreen, info);