avcodec/ffv1dec: Fix runtime error: signed integer overflow: 1550964438 + 1550964438 cannot be represented in type 'int'

Fixes: 1559/clusterfuzz-testcase-minimized-5048096079740928
Fixes: 1560/clusterfuzz-testcase-minimized-6011037813833728

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