avcodec/vp56: Check avctx->error_concealment before enabling EC

Fixes timeout with 847/clusterfuzz-testcase-5291877358108672
Fixes timeout with 850/clusterfuzz-testcase-5721296509861888

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