[BUGFIX] CAS frontend plugin crashes

Fixes: #61259
Change-Id: If5161b5c950891942265a29cb198e90eab4f0d83
Reviewed-on: http://review.typo3.org/32491
Reviewed-by: Bart Gijswijt <bart.gijswijt@ahk.nl>
Tested-by: Bart Gijswijt <bart.gijswijt@ahk.nl>
Reviewed-by: Xavier Perseguers <xavier@typo3.org>
Tested-by: Xavier Perseguers <xavier@typo3.org>