webrtc uses a int32_t like the existing code in ilbcdec

Fixes: signed integer overflow: 2080245063 + 257939661 cannot be represented in type 'int'
Fixes: 11037/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_ILBC_fuzzer-5682976612941824

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