[BUGFIX] Avoid double slashes in slug generation

When generating slugs it might happen, that double slashes
are generated. This basically results from both prepending
and appending slashes at the same time.

Resolves: #86479
Releases: master
Change-Id: I441fee333cd0b3fbad3dde1a130eb08356bd4c2d
Reviewed-on: https://review.typo3.org/58503


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>