avcodec/wmaenc: change commented assert to av_assert

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
jpeg2000dec: simplify jpeg2000_read_bitstream_packets()

23 lines less
less error prone as theres no more need to update *_size on every read

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