libavcodec/wmalosslessdec: Check the remaining bits

Fixes assertion failure
Fixes: 24ebfda03228b5cc1ef792608cfba458/signal_sigabrt_7ffff6ae7c37_6473_3fa8a111dbc752b1a7c411c5ab79aaa4.wma

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