popupMenu: close menu on destroy

This fixes a crash if a background menu is open
and the background changes.

https://bugzilla.gnome.org/show_bug.cgi?id=697432

js/ui/popupMenu.js

@@ -1157,6 +1157,7 @@ const PopupMenuBase = new Lang.Class({
     },
 
     destroy: function() {
+        this.close();
         this.removeAll();
         this.actor.destroy();