avformat/mvdec: Allocate extradata only once
Fixes: memleak
Fixes: 28686/clusterfuzz-testcase-minimized-ffmpeg_DEMUXER_fuzzer-5822961932173312
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>