avcodec/clearvideo: Only output a frame if one is coded in the packet

Fixes: Timeout (183 ms instead of about 20 sec)
Fixes: 3147/clusterfuzz-testcase-4870592182353920

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