Fix: reverse train direction command did not validate the given vehicle properly (#9947)

You could give a wagon in the chain to reverse (which makes no
functional sense ofc). In result, only parts of the vehicle were
reversing, leading to weird crashes.

src/train_cmd.cpp

@@ -1936,6 +1936,7 @@ CommandCost CmdReverseTrainDirection(DoCommandFlag flags, VehicleID veh_id, bool
 		}
 	} else {
 		/* turn the whole train around */
+		if (!v->IsPrimaryVehicle()) return CMD_ERROR;
 		if ((v->vehstatus & VS_CRASHED) || v->breakdown_ctr != 0) return CMD_ERROR;
 
 		if (flags & DC_EXEC) {