avcodec/mpegaudiodec: Eliminate many undefined operations

Fixes: 625/clusterfuzz-testcase-4574924406521856
Fixes: 626/clusterfuzz-testcase-4738718621499392

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