avcodec/mlz: Check offset before writing

Fixes: 0cee183a09bff5aa5108429717c35a4d/asan_heap-oob_1d99eca_3702_9ef60e80de79082a778d3d9ce8ef3b64.mp4
Affects no release

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