Decode (deprecated) BSON DBPointer to bson.dbref.DBRef.

def _get_ref(data, position, obj_end, opts, element_name):
    """Decode (deprecated) BSON DBPointer to bson.dbref.DBRef."""
    collection, position = _get_string(
        data, position, obj_end, opts, element_name)
    oid, position = _get_oid(data, position, obj_end, opts, element_name)
    return DBRef(collection, oid), position