jpeg2000: fix overflow in dequantization

Fixes decoding of file generated with:
ffmpeg -f lavfi -i smptehdbars=hd720 -pix_fmt rgb48 /tmp/o.jp2

Reviewed-by: Nicolas BERTRAND <nicoinattendu@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
