Fix #416: Division by zero for trains of 0 mass

In maximum achievable speed estimator

src/tbtr_template_vehicle.h

@@ -225,6 +225,6 @@ short DeleteTemplateReplacementsByGroupID(GroupID);
 
 void ReindexTemplateReplacements();
 
-int GetTemplateVehicleEstimatedMaxAchievableSpeed(const TemplateVehicle *tv, const int mass, const int speed_cap);
+int GetTemplateVehicleEstimatedMaxAchievableSpeed(const TemplateVehicle *tv, int mass, const int speed_cap);
 
 #endif /* TEMPLATE_VEH_H */

src/tbtr_template_vehicle_func.cpp

@@ -502,11 +502,13 @@ void UpdateAllTemplateVehicleImages()
 	_template_vehicle_images_valid = true;
 }
 
-int GetTemplateVehicleEstimatedMaxAchievableSpeed(const TemplateVehicle *tv, const int mass, const int speed_cap)
+int GetTemplateVehicleEstimatedMaxAchievableSpeed(const TemplateVehicle *tv, int mass, const int speed_cap)
 {
 	int max_speed = 0;
 	int acceleration;
 
+	if (mass < 1) mass = 1;
+
 	do
 	{
 		max_speed++;

src/train.h

@@ -520,6 +520,6 @@ inline int GetTileMarginInFrontOfTrain(const Train *v)
 int GetTrainStopLocation(StationID station_id, TileIndex tile, Train *v, bool update_train_state, int *station_ahead, int *station_length);
 
 int GetTrainRealisticAccelerationAtSpeed(const int speed, const int mass, const uint32 cached_power, const uint32 max_te, const uint32 air_drag, const RailType railtype);
-int GetTrainEstimatedMaxAchievableSpeed(const Train *train, const int mass, const int speed_cap);
+int GetTrainEstimatedMaxAchievableSpeed(const Train *train, int mass, const int speed_cap);
 
 #endif /* TRAIN_H */

src/train_cmd.cpp

@@ -7320,11 +7320,13 @@ int GetTrainRealisticAccelerationAtSpeed(const int speed, const int mass, const
 	return acceleration;
 }
 
-int GetTrainEstimatedMaxAchievableSpeed(const Train *train, const int mass, const int speed_cap)
+int GetTrainEstimatedMaxAchievableSpeed(const Train *train, int mass, const int speed_cap)
 {
 	int max_speed = 0;
 	int acceleration;
 
+	if (mass < 1) mass = 1;
+
 	do
 	{
 		max_speed++;