avcodec/vp56: Clear dimensions in case of failure in the middle of a resolution change

Similar code is used elsewhere in vp56 to force a more complete reinit in the future.
Fixes null pointer dereference
Fixes: 707/clusterfuzz-testcase-4717453097566208

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