avcodec/clearvideo: Fix multiple runtime error: left shift of negative value -1024

Fixes: 1360/clusterfuzz-testcase-minimized-5606472043986944

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