avcodec/vp56: Require a correctly decoded frame before using vp56_conceal_mb()

Fixes timeout with 700/clusterfuzz-testcase-5660909504561152
Fixes timeout with 702/clusterfuzz-testcase-4553541576294400

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