avcodec/adpcm: fix sample count for stereo SBPRO3

Fixes use of uninitialized memory
Fixes: msan_uninit-mem_7f9b9902ed90_7462_new_alaw.voc
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
