avcodec/vp6: clear dimensions on failed resolution change in vp6_parse_header()

Fixes: 807/clusterfuzz-testcase-6470061042696192
Fixes null pointer dereference

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