[BUGFIX] Fatal error during install process

In the install tool there is no backend user available,
thus this needs to be checked before calling the writelog() method.

Change-Id: I16494e0ba01147a5111101e9a76cf44418c1c5fb
Fixes: #32666
Reviewed-on: http://review.typo3.org/7363
Reviewed-by: Oliver Hader
Tested-by: Oliver Hader