Open redirect in Serendipity (exit.php)

Serendipity contains a script named `exit.php` that can be directly accessed. When crafting an hyperlink pointing to this page with the parameter `url` containing a base64-encoded URL, it will redirect the user to this URL.

<?php
// [...]
if (isset($_GET['url_id']) && !empty($_GET['url_id']) && isset($_GET['entry_id']) && !empty($_GET['entry_id'])) {
// [...]
} elseif (isset($_GET['url']) && !empty($_GET['url'])) {
    // No entry-link ID was submitted. Possibly a spammer tried to mis-use the script to get into the …