Fixes: signed integer overflow: -1801695444 + -830224908 cannot be represented in type 'int'
Fixes: 17995/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_TRUESPEECH_fuzzer-5648084880588800

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