Admin extension page not displayed when there is not extensions

The problem described on http://forum.civicrm.org/index.php?topic=21136.0 , I got the same. There is a diff attached.

To resume the problem is CRM_Core_Extensions::getExtensions returns an empty array, so the condition "if ( self::$_extensions )" in CRM_Core_Admin_Extension::browse is false, which is interpreted as "the extension directory is not set" in the template file.

Note: it works in my CiviCRM copy, but check it's correct and there is no better way to solve it since I'm a CiviCRM beginner.