[BUGFIX] Check if folder is within the filemount

Before using the filemount base folder as root for the
folder->getReadablePath() check if the folder is within the filemount.

Change-Id: I8a4436fb2a0e512c7d2d1fc50b9d08febae8b256
Resolves: #73735
Releases: master, 7.6, 6.2
Reviewed-on: https://review.typo3.org/47793


Reviewed-by: Nicole Cordes <typo3@cordes.co>
Tested-by: Nicole Cordes <typo3@cordes.co>
Reviewed-by: Lorenz Ulrich <lorenz.ulrich@visol.ch>
Tested-by: Lorenz Ulrich <lorenz.ulrich@visol.ch>
Reviewed-by: Frans Saris <franssaris@gmail.com>
Tested-by: Frans Saris <franssaris@gmail.com>