[BUGFIX] Prevent double slash in isWithin check

If the root folder is used as folder identifier (/) no slash is appended
so that isFirstPartOfStr works correctly in the isWithin check in
LocalDriver.

Resolves: #55740
Releases: 6.2
Change-Id: I2af1cd3759656502ea84887828dfb222f0a07b64
Reviewed-on: https://review.typo3.org/27379
Reviewed-by: Markus Klein
Reviewed-by: Markus Kasten
Reviewed-by: Steffen Ritter
Tested-by: Steffen Ritter