avcodec/sonic: Use unsigned for predictor_k to avoid undefined behavior

Fixes: signed integer overflow: -1094995529 * 24 cannot be represented in type 'int'
Fixes: 44436/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_SONIC_fuzzer-4874459459223552

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg


Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 28008bf9)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>