Update extension_customizer.php

[29-May-2014 20:45:55 UTC] PHP Notice:  Undefined property: ReduxFramework_extension_customizer::$args in /Applications/MAMP/htdocs/wordpress/wp-content/plugins/redux-framework/ReduxCore/extensions/customizer/extension_customizer.php on line 421
Merge pull request #1286 from paulthecoder/patch-2

Update extension_customizer.php