wavpack: use bytestream2 in wavpack_decode_block

Prevent most out of buffer reads.

Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org
wavpack: check packet size early

Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org