avcodec/cdgraphics: Clear first frame only once

frame_number will not increase if nothing is output

Fixes: Timeout
Fixes: 9057/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_CDGRAPHICS_fuzzer-4844661498707968

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg


Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>