If a URL like

http://localhost/phpbb3/../../../index.php

is supplied to clean_path(), the method removes more slashes than it obviously should. The above URL results in this cleaned path:

http:/index.php

That new, cleaned URL is obviously incorrect and does not follow the URI scheme anymore.
I'm not sure if clean_path shouldn't be used for URLs or if we should just make sure that the slashes after the URI scheme are not removed.