shell: Fix segfault from trying to access a destroyed popup shell surface

The shell_destroy_shell_surface function only set the backing resource to
NULL, leaving an unusable surface in the popup_grab list until the surface's
fading animation finished and it could be freed. This caused a segfault if
the shell tried to forcibly break the grab during that time interval due to
the compositor losing the keyboard focus.

https://bugs.freedesktop.org/show_bug.cgi?id=77072

desktop-shell/shell.c

@@ -3050,6 +3050,8 @@ shell_destroy_shell_surface(struct wl_resource *resource)
 {
 	struct shell_surface *shsurf = wl_resource_get_user_data(resource);
 
+	if (!wl_list_empty(&shsurf->popup.grab_link))
+		remove_popup_grab(shsurf);
 	shsurf->resource = NULL;
 }