Fix potential crash when fullscreen view unmaps

It happened when a view is a grandchild or deeper of the workspace, is
fullscreen, and unmaps. The workspace would not be included in the
transaction and its pointer to the fullscreen view was left dangling.

sway/tree/view.c

@@ -542,14 +542,16 @@ void view_map(struct sway_view *view, struct wlr_surface *wlr_surface) {
 struct sway_container *view_unmap(struct sway_view *view) {
 	wl_signal_emit(&view->events.unmap, view);
 
+	wl_list_remove(&view->surface_new_subsurface.link);
+	wl_list_remove(&view->container_reparent.link);
+
 	if (view->is_fullscreen) {
 		struct sway_container *ws = container_parent(view->swayc, C_WORKSPACE);
 		ws->sway_workspace->fullscreen = NULL;
+		container_destroy(view->swayc);
+		return ws;
 	}
 
-	wl_list_remove(&view->surface_new_subsurface.link);
-	wl_list_remove(&view->container_reparent.link);
-
 	return container_destroy(view->swayc);
 }