seg_data structure should be zeroed if segmentation is not enabled

This is described in section 5.9.14 of the AV1 specification (which describes how to read segmentation data in parse_frame_hdr). In the case that segmentation is not enabled, but hdr->primary_ref_frame is not none, dav1d is not currently zeroing the segmentation data and instead is incorrectly copying it from the primary ref frame.