Fix #10926: New free wagons no longer split by type. (#11507)

NormalizeTrainVehInDepot() should only be called if an engine was built, not for wagons.

src/train_cmd.cpp

@@ -689,6 +689,7 @@ static CommandCost CmdBuildRailWagon(DoCommandFlag flags, TileIndex tile, const
 /** Move all free vehicles in the depot to the train */
 void NormalizeTrainVehInDepot(const Train *u)
 {
+	assert(u->IsEngine());
 	for (const Train *v : Train::Iterate()) {
 		if (v->IsFreeWagon() && v->tile == u->tile &&
 				v->track == TRACK_BIT_DEPOT) {