avcodec/wmalosslessdec: completely initialize contains_subframe

Fixes use of uninitialized memory
Fixes: msan_uninit-mem_7f60c46325d7_6415_luckynight.wma
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>