avcodec/wmalosslessdec: shrink output on error so no uninitialized data is returned

Fixes use of uninitialized memory
partly fixes: msan_uninit-mem_7f7834b6a530_6473_luckynight-partial.wma
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
