avcodec/jpeg2000dec: zero Jpeg2000QuantStyle structure before use in get_qcd()

Fixes: msan_uninit-mem_7f50b84aac30_6823_mjp2.mov
Fixes use of uninitialized memory
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>