avcodec/wmaprodec: Check for overread in decode_packet()

Fixes assertion failure
Fixes: 0256e92df2df7e933b43a2c70e4c8040/signal_sigabrt_7ffff6ae7cc9_1358_999ac18684788221490757582ce9af84.wma

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
