Fix crash in seat code

Container will be NULL if launching swaylock.

sway/input/seat.c

@@ -669,7 +669,7 @@ void seat_set_focus_warp(struct sway_seat *seat,
 	// If we've focused a floating container, bring it to the front.
 	// We do this by putting it at the end of the floating list.
 	// This must happen for both the pending and current children lists.
-	if (container_is_floating(container)) {
+	if (container && container_is_floating(container)) {
 		list_move_to_end(container->parent->children, container);
 		if (container_has_ancestor(container, container->current.parent)) {
 			list_move_to_end(container->parent->current.children, container);