animations: Guard against empty animations

Our animation code not only relies on the animation being loaded
(which we handle), but also on having at least one frame - otherwise
the computation of the next frame index will turn up NaN through
division by zero. Guard against this case by treating empty animations
as not loaded.

https://bugzilla.gnome.org/show_bug.cgi?id=774805

js/ui/animation.js

@@ -67,7 +67,7 @@ const Animation = new Lang.Class({
     },
 
     _animationsLoaded: function() {
-        this._isLoaded = true;
+        this._isLoaded = this._animations.get_n_children() > 0;
 
         if (this._isPlaying)
             this.play();