osdWindow: Don't tween to undefined

Check that we got a valid level before setting the level bar to it, to
prevent a Tweener warning.

js/ui/osdWindow.js

@@ -125,6 +125,7 @@ const OsdWindow = new Lang.Class({
 
     setLevel: function(level) {
         this._level.actor.visible = (level != undefined);
+        if (level) {
             if (this.actor.visible)
                 Tweener.addTween(this._level,
                                  { level: level,
@@ -132,6 +133,7 @@ const OsdWindow = new Lang.Class({
                                    transition: 'easeOutQuad' });
             else
                 this._level.level = level;
+        }
     },
 
     show: function() {