avformat/xmv: Discard remainder of packet on error

Fixes infinite loop
Fixes: 9c48ae2680c5f23bca3d20ff0f325fd8/asan_generic_4c254d_1374_993f1e5967dd6f844b8d72f978ce2a6c.pss

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