Codechange: Get count of road vehicles by summing company group statistics data in small UFO handler. (#11690)

This avoids needing to iterate the complete vehicle pool. Company group statistics are maintained elsewhere already.

The vehicle pool is still iterated later to find the nth random road vehicle.

src/disaster_vehicle.cpp

@@ -334,8 +334,8 @@ static bool DisasterTick_Ufo(DisasterVehicle *v)
 		v->state = 1;
 
 		uint n = 0; // Total number of targetable road vehicles.
-		for (const RoadVehicle *u : RoadVehicle::Iterate()) {
-			if (u->IsFrontEngine()) n++;
+		for (const Company *c : Company::Iterate()) {
+			n += c->group_all[VEH_ROAD].num_vehicle;
 		}
 
 		if (n == 0) {