Redirect to installer might be invalid when accessing subfolder

After PHPBB3-12785, we now prefer REQUEST_URI over PHP_SELF. This has actually forced an issue that would have appeared if someone would have disabled PHP_SELF beforehand:

When a user accesses example.com/foo/bar/, where example.com/foo/bar/ is the forum root, they'd be redirected to the installer. Unfortunately, the redirect in common.php does run dirname() on REQUEST_URI, which results in dirname() on /foo/bar/, essentially ending up with /foo. This is an oversight and dirname should not be run if the URI ends with a slash.