Fixes: 10651/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_ILBC_fuzzer-5202341540659200
Fixes: signed integer overflow: -1707705920 - 1703592888 cannot be represented in type 'int'

This tries to follow the webrtc code. For example using cliping and 64 bit as in WebRtcSpl_DotProductWithScale()
and not doing so in other places.
I could not find anything in rfc3951 and the reference code inside which would
explain what to do in these corner cases.

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