xwayland: avoid crash on repeated server_finish_display() call

This function may end up being called more than once if the Xwayland
binary does not exist on the system.

xwayland/server.c

@@ -164,6 +164,7 @@ static void server_finish_display(struct wlr_xwayland_server *server) {
 	}
 
 	wl_list_remove(&server->display_destroy.link);
+	wl_list_init(&server->display_destroy.link);
 
 	if (server->display == -1) {
 		return;