Fix infinite loop when system if offline due to pending upgrade

offline.php

@@ -14,7 +14,7 @@
     vim: expandtab sw=4 ts=4 sts=4:
 **********************************************************************/
 require_once('client.inc.php');
-if($cfg && !$cfg->isHelpDeskOffline()) { 
+if(is_object($ost) && $ost->isSystemOnline()) {
     @header('Location: index.php'); //Redirect if the system is online.
     include('index.php');
     exit;