avcodec/clearvideo: Check buf_size before decoding frame

Fixes; Timeout
Fixes: 1826/clusterfuzz-testcase-minimized-5728569256837120

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