avcodec/celp_filters: Fix runtime error: signed integer overflow: 1892453989 + 381702783 cannot be represented in type 'int'

Fixes: 1766/clusterfuzz-testcase-minimized-6562020075765760

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