avformat/mxg: Use memmove()

Fixes undefined behavior
Fixes: 1700002963a49da13542e0726b7bb758/unknown_unknown_292_658_cov_2141972066_m1.mxg

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