xwm: remove configure_source on destroy

It would lead to use-after-free if there was a pending idle callback to
weston_wm_window_configure() when the weston_wm_window gets destroyed. Make
sure the callback will not fire.

Found by inspection.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>

xwayland/window-manager.c

@@ -1514,6 +1514,8 @@ weston_wm_window_destroy(struct weston_wm_window *window)
 
 	weston_output_weak_ref_clear(&window->legacy_fullscreen_output);
 
+	if (window->configure_source)
+		wl_event_source_remove(window->configure_source);
 	if (window->repaint_source)
 		wl_event_source_remove(window->repaint_source);
 	if (window->cairo_surface)