[BUGFIX] Ensure new deep dirs don't contain double slashes

The method of GeneralUtility::mkdir_deep has two parameters, which are
concatenated. This could end up in a double slashes. To prevent that,
the parameters are modified to concat them with only one slash.

Additionally fix a Bug with double slash in FileStreamWrapper.

Resolves: #63920
Releases: master, 6.2
Change-Id: I2d15f6c33550f9045db9b7730f7b074ae54a18dc
Reviewed-on: http://review.typo3.org/36399


Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Tested-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>