avcodec/adpcm: Fix adpcm_ima_wav padding

Fixes out of array read
Fixes: f29f134ea5f5590df554a7733294a587/asan_stack-oob_309d14e_9188_ea01743d6355aff20530f3d4cdaa841a.wav

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