animation: Disable spinner animations when actor is destroyed

There's nothing to animate anymore, just a source for warnings when
trying to access a destroyed object.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/362

js/ui/animation.js

@@ -104,6 +104,11 @@ var Spinner = new Lang.Class({
         this._animate = animate;
     },
 
+    _onDestroy() {
+        this._animate = false;
+        this.parent();
+    },
+
     play() {
         Tweener.removeTweens(this.actor);