Exit frame data decoding for OBU_FRAME if show_existing_frame=1
Exit frame data decoding for OBU_FRAME if show_existing_frame=1

This is identical to what libaom does for such OBUs. It prevents us
from doing block decoding, which doesn't make sense for such OBUs,
and would result in using an uninitialized frame header during the
decoding process. Fixes #94.

videolan/dav1d