avcodec/fic: Fixes signed integer overflow

Fixes: runtime error: signed integer overflow: 1037142357 + 1227025305 cannot be represented in type 'int'
Fixes: 3024/clusterfuzz-testcase-minimized-5885660323905536

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