avcodec/jpeg2000dec: merge bytestream2 reading from j2kdec

This fixes at least one out of array access

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
j2kdec: cosmetics from jpeg2000dec

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>