avformat/mpc8: Check size before implicitly converting to int
Fixes: Timeout
Fixes: 28551/clusterfuzz-testcase-minimized-ffmpeg_dem_MPC8_fuzzer-6229183210586112
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>