Fix #9802: Assertion failure in BuildRailWaypointWindow constructor

src/rail_gui.cpp

@@ -2011,6 +2011,7 @@ struct BuildRailWaypointWindow : PickerWindowBase {
 		this->FinishInitNested(TRANSPORT_RAIL);
 
 		matrix->SetCount(_waypoint_count);
+		if (_cur_waypoint_type >= _waypoint_count) _cur_waypoint_type = 0;
 		matrix->SetClicked(_cur_waypoint_type);
 	}