avcodec/cavsdec: Fix runtime error: signed integer overflow: 31 + 2147483640 cannot be represented in type 'int'

Fixes: 1506/clusterfuzz-testcase-minimized-5401272918212608

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