libweston/input: Assert if we're still having a notify listener installed

Tracking correctly previous events shouldn't corrupt the surface destroy
signal list. This enforces that by ensuring that we wouldn't have
a .notify wl_listener still being set (which shouldn't happen if we do
eventually get a focus_in event that clears it out).

Suggested-by: Alexandros Frantzis <alexandros.frantzis@collabora.com>
Signed-off-by: Marius Vlad <marius.vlad@collabora.com>

libweston/input.c

@@ -2329,6 +2329,7 @@ notify_keyboard_focus_out(struct weston_seat *seat)
 	if (focus) {
 		seat->use_saved_kbd_focus = true;
 		seat->saved_kbd_focus = focus;
+		assert(seat->saved_kbd_focus_listener.notify == NULL);
 		seat->saved_kbd_focus_listener.notify =
 			destroy_device_saved_kbd_focus;
 		wl_signal_add(&focus->destroy_signal,