[BUGFIX] Suhosin triggers warnings in Install Tool

When Suhosin runs in simulation mode, violations are logged
as usual, but nothing is blocked or removed from the request.

Add an extra check to isSuhosinLoaded, to prevent unneeded
warnings.

Resolves: #55886
Releases: master, 6.2
Change-Id: Idd07872d31d51ed3266fcf330e825187656167a8
Reviewed-on: http://review.typo3.org/27822
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Reviewed-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Tested-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>