(valgrind trace, line-numbers from mixbus+6.0-190-g0ec6bc35a)
This may happen initially for unconnected graph nodes,
e.g. Foldback Busses from ARDOUR::Session::post_engine_init().
==29797== Conditional jump or move depends on uninitialised value(s)
==29797== at 0x6167D3F: trace_terminal(boost::shared_ptr<ARDOUR::Route>, boost::shared_ptr<ARDOUR::Route>, bool) (session.cc:2174)
==29797== by 0x6167D99: trace_terminal(boost::shared_ptr<ARDOUR::Route>, boost::shared_ptr<ARDOUR::Route>, bool) (session.cc:2174)
==29797== by 0x6167D99: trace_terminal(boost::shared_ptr<ARDOUR::Route>, boost::shared_ptr<ARDOUR::Route>, bool) (session.cc:2174)
==29797== by 0x616890D: ARDOUR::Session::resort_routes_using(boost::shared_ptr<std::__cxx11::list<boost::shared_ptr<ARDOUR::Route>, std::allocator<boost::shared_ptr<ARDOUR::Route> > > >) (session.cc:2289)