Fix crash when moving view across outputs

It was incorrectly determining that the container being moved and the
destination had the same parent, which resulted in tree corruption. Both
parents can be NULL but the containers may belong to different
workspaces.

To reproduce, create layout H[V[view] view] in one workspace then move a
view left or right from another output into that workspace.

sway/commands/move.c

@@ -117,7 +117,8 @@ static void container_move_to_container_from_direction(
 		struct sway_container *container, struct sway_container *destination,
 		enum movement_direction move_dir) {
 	if (destination->view) {
-		if (destination->parent == container->parent) {
+		if (destination->parent == container->parent &&
+				destination->workspace == container->workspace) {
 			wlr_log(WLR_DEBUG, "Swapping siblings");
 			list_t *siblings = container_get_siblings(container);
 			int container_index = list_find(siblings, container);