[BUGFIX] Check if mount page exist before fetching icon

To avoid fatal errors it must be checked if the mount pages
really exist before fetching its icon.

Resolves: #77663
Releases: master, 7.6
Change-Id: Icdb6009c6304096078e7b5bdba733f997a75e9f2
Reviewed-on: https://review.typo3.org/49669


Tested-by: Bamboo TYPO3com <info@typo3.com>
Reviewed-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: Andreas Fernandez <typo3@scripting-base.de>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>