Fix UAF in comps_objmrtree_unite function

The added field is not used at all in many places and it is probably the
left-over of some copy-paste.
COMPS_ObjRTreeData * comps_objrtree_data_create_n(char *key, size_t keylen,
                                                  COMPS_Object *data) {
    COMPS_ObjRTreeData * rtd;
    rtd = __comps_objrtree_data_create(key, keylen, data);
    return rtd;
}

COMPS_ObjRTreeData * comps_objrtree_data_create_n(char *key, size_t keylen,
                                                  COMPS_Object *data) {
    COMPS_ObjRTreeData * rtd;
    rtd = __comps_objrtree_data_create(key, keylen, data);
    return rtd;
}
