This prevents out of array reads. An alternative solution would be
to check the index but this would require several checks in the
inner loops

Yet another alternative would be to change the index reset logic
but this likely would introduce a difference to the binary decoder

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>