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.
void comps_objrtree_set(COMPS_ObjRTree *rt, char *key, COMPS_Object *data) {
    __comps_objrtree_set(rt, key, strlen(key), comps_object_incref(data));
}

void comps_objrtree_set(COMPS_ObjRTree *rt, char *key, COMPS_Object *data) {
    __comps_objrtree_set(rt, key, strlen(key), comps_object_incref(data));
}
