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_RTree * comps_rtree_create(void* (*data_constructor)(void*),
                                 void* (*data_cloner)(void*),
                                 void (*data_destructor)(void*)) {
    COMPS_RTree *ret;
    if ((ret = malloc(sizeof(COMPS_RTree))) == NULL)
        return NULL;
    ret->subnodes = comps_hslist_create();
    comps_hslist_init(ret->subnodes, NULL, NULL, &comps_rtree_data_destroy_v);
    if (ret->subnodes == NULL) {
        free(ret);
        return NULL;
    }
    ret->data_constructor = data_constructor;
    ret->data_cloner = data_cloner;
    ret->data_destructor = data_destructor;
    return ret;
}

COMPS_RTree * comps_rtree_create(void* (*data_constructor)(void*),
                                 void* (*data_cloner)(void*),
                                 void (*data_destructor)(void*)) {
    COMPS_RTree *ret;
    if ((ret = malloc(sizeof(COMPS_RTree))) == NULL)
        return NULL;
    ret->subnodes = comps_hslist_create();
    comps_hslist_init(ret->subnodes, NULL, NULL, &comps_rtree_data_destroy_v);
    if (ret->subnodes == NULL) {
        free(ret);
        return NULL;
    }
    ret->data_constructor = data_constructor;
    ret->data_cloner = data_cloner;
    ret->data_destructor = data_destructor;
    return ret;
}
