Don't wrap tab info in Tab if Tab is not defined

safari/common.js

 /*
  * This file is part of Adblock Plus <http://adblockplus.org/>,
  * Copyright (C) 2006-2013 Eyeo GmbH
  *
  * Adblock Plus is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 3 as
  * published by the Free Software Foundation.
  *
  * Adblock Plus is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 58 matching lines @@
     _wrapListener: function(listener)
     {
       return function(event)
       {
         if (event.name.indexOf("request-") != 0)
           return;
 
         var sender = {};
         var dispatcher;
 
-        if ("SafariBrowserTab" in window && event.target instanceof SafariBrowserTab)
+        if ("Tab" in window && "SafariBrowserTab" in window && event.target instanceof SafariBrowserTab)
         {
           dispatcher = event.target.page;
           sender.tab = new Tab(event.target);
         }
         else
         {
           dispatcher = event.target.tab;
           sender.tab = null;
         }
 
@@ skipping 130 matching lines @@
   /* API */
 
   ext = {
     getURL: function(path)
     {
       return safari.extension.baseURI + path;
     },
     i18n: new I18n()
   };
 })();