scsi_debug: vfree is null safe so drop the check

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Acked-by: Douglas Gilbert <dgilbert@interlog.com>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: James Bottomley <JBottomley@Odin.com>