[BUGFIX] Make sure relative URL receives a proper host part for errorpage

If the code given for PageNotFound_handling is a relative URL, the host
determination gets skipped, resulting in a redirect of the markup of
the page instead a proper redirect.

Change-Id: Id9c26cba813c212438368cee031673dc59db360b
Resolves: #81267
Relates: #81031
Releases: master, 8.7
Reviewed-on: https://review.typo3.org/52859


Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Mathias Brodala <mbrodala@pagemachine.de>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>