avcodec/alsdec: Fix mlz memleak

Fixes: 0cee183a09bff5aa5108429717c35a4d/asan_heap-oob_1d99eca_3702_172c75af9868d4c2556a79cc2413f4cc.mp4

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