avcodec/vp56: Fix sign typo

Fixes: 664/clusterfuzz-testcase-4917047475568640

The change to fate is due to a truncated last frames which is now detected as damaged.

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