avcodec/truemotion1: Fix multiple runtime error: signed integer overflow: 1246906962 * 2 cannot be represented in type 'int'

Fixes: 1616/clusterfuzz-testcase-minimized-5119196578971648

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