avcodec/fic: Check coefficients

Fixes: signed integer overflow: 1258291200 * 2 cannot be represented in type 'int'
Fixes: 1413/clusterfuzz-testcase-minimized-5923451770503168

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