avcodec/zmbv: Check decomp_size

Fixes: OOM
Fixes: 2710/clusterfuzz-testcase-minimized-4750001420894208

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Reviewed-by: Tomas Härdin <tjoppen@acc.umu.se>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
