avcodec/jpeg2000dec: get_qcx, fix stack and heap overwrites

This is likely remotely exploitable
Fix ported from j2kdec
No uptodate FFmpeg release should be affected by this

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