flacdec: don't call allocate_buffers() in init_thread_copy() when s->max_blocksize is 0

This is normal, and happens when no extradata was available at decoder
initialization.

Signed-off-by: Paul B Mahol <onemda@gmail.com>
jpeg2000dec: Support 4 tile parts

Fixes out of array accesses

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