As is the decoder will never stop, it will cause an infinite loop. The RFC seems only
to speak of non empty packets so endlessly generating noise from the last empty flush
packets seems wrong.

Fixes: infinite loop
Fixes: 18333/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_COMFORTNOISE_fuzzer-5668481831272448

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