avcodec/ffv1dec_template: Fix runtime error: signed integer overflow: 202 + 2147483615 cannot be represented in type 'int'

Fixes: 1748/clusterfuzz-testcase-minimized-6690208340770816

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