avcodec/vb: Check vertical GMC component before multiply

Fixes: runtime error: signed integer overflow: 8224 * 663584 cannot be represented in type 'int'
Fixes: 2393/clusterfuzz-testcase-minimized-6128334993883136

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