avcodec/clearvideo: Fix runtime error: signed integer overflow: 181 * 18050756 cannot be represented in type 'int'

Fixes: 1417/clusterfuzz-testcase-minimized-6606778030620672

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