avcodec/tscc2: Skip duplicate frames

This turns CFR duplicated frames into skiped frames

Fixes: Timeout
Fixes: 1719/clusterfuzz-testcase-minimized-6375090079924224

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