Use $_SERVER['PATH_TRANSLATED'] when $_SERVER['SCRIPT_FILENAME'] is not set

In some configurations (e.g. Nginx + PHP via php-fpm) $_SERVER['SCRIPT_FILENAME'] is not set and script path can by found in $_SERVER['PATH_TRANSLATED']. This modification makes plugin to check presence of both server variables to make it work in less common configurations. Without it, access to some wp-admin scripts like admin-ajax.php is rejected.