Fix inconsistent session-check

Fix inconsistent session-check

under some circumstances, e.g. session-exit it is possible to to have
inconsistent information
    ARDOUR_UI::instance()->session_loaded == true
 && ARDOUR_UI::instance()->the_session() == NULL

This lead to a crash on MacOS/X. stopping the engine at exit may destory
an Aggregate Device, leading to DeviceListChanged which in turn updates
the engine-dialog..
 else if (ARDOUR_UI::instance()->session_loaded) {
   float active_sr = ARDOUR_UI::instance()->the_session()->nominal_sample_rate ();