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.
inline void comps_mrtree_data_destroy_v(void * rtd) {
    comps_mrtree_data_destroy((COMPS_MRTreeData*)rtd);
}

inline void comps_mrtree_data_destroy_v(void * rtd) {
    comps_mrtree_data_destroy((COMPS_MRTreeData*)rtd);
}
