appMenu: Make show() a no-op in locked state

The app menu is hidden when entering the lock screen, however it
might be shown again while the lock is still in place - we don't
want this ever to be the case, so make show() a no-op while the
screen is locked.

https://bugzilla.gnome.org/show_bug.cgi?id=682475

js/ui/panel.js

@@ -285,7 +285,7 @@ const AppMenuButton = new Lang.Class({
     },
 
     show: function() {
-        if (this._visible)
+        if (this._visible || Main.screenShield.locked)
             return;
 
         this._visible = true;