[BUGFIX] Check opendir result in fixPermissions

Checks opendir result if it is a resource before passing it
to readdir.

Resolves: #47981
Releases: master, 7.6
Change-Id: I45ca844c7ab353a28dc7002ec206bcb054b01c96
Reviewed-on: https://review.typo3.org/48072


Reviewed-by: Nicole Cordes <typo3@cordes.co>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>