If an extension is missing and a link to url.php is displayed on the login page, a click on this link just calls back phpMyAdmin. I was able to fix in QA_3_5 by replacing common.inc.php by core.lib.php in url.php, but this did not work in master. After the require_once, $_GET['url'] is empty.
See https://github.com/phpmyadmin/phpmyadmin/pull/135