avcodec/truemotion2: Move skip computation after checks

Fixes: runtime error: signed integer overflow: 630067357 * 4 cannot be represented in type 'int'
Fixes: 2233/clusterfuzz-testcase-minimized-5943031318446080

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