panelMenu: Fix exception when destroying menuless button

There's explicit API to create PanelMenu.Buttons with no menu, so
guard against this case in destroy().

https://bugzilla.gnome.org/show_bug.cgi?id=686763

js/ui/panelMenu.js

@@ -214,6 +214,7 @@ const Button = new Lang.Class({
     destroy: function() {
         this.actor._delegate = null;
 
+        if (this.menu)
             this.menu.destroy();
         this.actor.destroy();