Fix crash in workspace_wrap_children

When workspace_wrap_children is called on a workspace which has a
fullscreen child and the fullscreen child is a direct child of the
workspace, sway would crash.

The workspace's fullscreen pointer is unset when the fullscreen
container is detached and applied again when added to a parent, but in
this case the parent hadn't yet been added to the workspace which meant
con->workspace was NULL.

The fix makes container_handle_fullscreen_reparent return if there's no
workspace, and the fullscreen pointer is reapplied in
workspace_wrap_children.

sway/tree/container.c

@@ -1063,7 +1063,8 @@ list_t *container_get_current_siblings(struct sway_container *container) {
 }
 
 void container_handle_fullscreen_reparent(struct sway_container *con) {
-	if (!con->is_fullscreen || con->workspace->fullscreen == con) {
+	if (!con->is_fullscreen || !con->workspace ||
+			con->workspace->fullscreen == con) {
 		return;
 	}
 	if (con->workspace->fullscreen) {

sway/tree/workspace.c

@@ -557,6 +557,7 @@ struct sway_container *workspace_find_container(struct sway_workspace *ws,
 }
 
 struct sway_container *workspace_wrap_children(struct sway_workspace *ws) {
+	struct sway_container *fs = ws->fullscreen;
 	struct sway_container *middle = container_create(NULL);
 	middle->layout = ws->layout;
 	while (ws->tiling->length) {
@@ -565,6 +566,7 @@ struct sway_container *workspace_wrap_children(struct sway_workspace *ws) {
 		container_add_child(middle, child);
 	}
 	workspace_add_tiling(ws, middle);
+	ws->fullscreen = fs;
 	return middle;
 }