avcodec/alsdec: Fix reading 0 mantisse bits

Fixes assertion failure
Fixes: 848c24abc1721c9e3d1ba7bfee8d9fcc/asan_heap-oob_1d99eca_3709_567bba70d67e7d62714dcf56f26fb1da.mp4

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