background: Do not queue idle when cancelled

A cancelled cancellable means that the background was destroyed,
so we shouldn't queue an idle or emit the 'loaded' signal anymore.

https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/5337

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2268>

js/ui/background.js

@@ -333,6 +333,8 @@ var Background = GObject.registerClass({
             return;
 
         this.isLoaded = true;
+        if (this._cancellable?.is_cancelled())
+            return;
 
         let id = GLib.idle_add(GLib.PRIORITY_DEFAULT, () => {
             this.emit('loaded');