login redirect URL doesn't check for redirecting to a redirect

[LegalizeAdulthood]

To reproduce:

1. Click login link
2. Instead of logging in, click the login link again.
3. The redirect is cascaded instead of just being a single level, e.g. https://manx-docs.org/login.php?redirect=%2Flogin.php%3Fredirect%3D%252Flogin.php%253Fredirect%253D%25252Flogin.php%25253Fredirect%25253D%2525252Fhelp.php