[BUGFIX] Ensure MountPoint parameter contains valid characters

When using ?MP=12-13,345-673 some could also add a
letter or a "/" symbol in the GET parameter, effectively
showing a 503 instead of 404 page. This change ensures
that $TSFE->MP only contains numbers, commas or dashes.

Resolves: #94045
Releases: master, 10.4
Change-Id: I9d47153495701eb36bd9eb0c9936c0956b075560
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/68977


Tested-by: core-ci <typo3@b13.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Nikita Hovratov <nikita.h@live.de>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Nikita Hovratov <nikita.h@live.de>
Reviewed-by: Benni Mack <benni@typo3.org>