cfhd: fix off-by-one error in level check

This fixes out-of-bounds writes causing segmentation faults.

Found-by: Piotr Bandurski <ami_stuff@o2.pl>
Reviewed-by: Kieran Kunhya <kierank@obe.tv>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
