[BUGFIX] Ignore open_basedir warning for files outside the website root

Suppress warnings generated by getFilenameFromMainDir() when it needs
to check file paths outside the website root and open_basedir
is in effect.

Resolves: #81024
Releases: master, 8.7
Change-Id: Ia339c907577547374545c0a94393f698828523c7
Reviewed-on: https://review.typo3.org/52614


Reviewed-by: Stephan Großberndt <stephan@grossberndt.de>
Tested-by: Stephan Großberndt <stephan@grossberndt.de>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Henrik Elsner <helsner@dfau.de>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>