BackgroundCache: plug an Animation object leak

We need to return early in case the animation file is the same,
otherwise we'll create a new Animation object and leak the previous
one.

https://bugzilla.gnome.org/show_bug.cgi?id=739252

js/ui/background.js

@@ -172,6 +172,7 @@ const BackgroundCache = new Lang.Class({
                 }));
                 GLib.Source.set_name_by_id(id, '[gnome-shell] params.onLoaded');
             }
+            return;
         }
 
         let animation = new Animation({ file: params.file });