avcodec/mpeg4videodec: Check for multiple VOL headers

Fixes multiple: runtime error: signed integer overflow: 2147115008 + 413696 cannot be represented in type 'int'
Fixes: 1723/clusterfuzz-testcase-minimized-5309409372667904
Fixes: 1727/clusterfuzz-testcase-minimized-5900685306494976
Fixes: 1737/clusterfuzz-testcase-minimized-5922321338466304

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