(svn r9981) -Fix: fixed a rare event that could cause autoreplace to run out of money and generate an error (spotted by elmex and UndernotBuilder)

This happened if the new engine was a dualheaded engine and there was money to replace and refit the front, but not the rear
  It also adds the cost of refitting the rear end of dualheaded engines to the cost animations (display only)

src/autoreplace_cmd.cpp

@@ -171,7 +171,12 @@ static int32 ReplaceVehicle(Vehicle **w, byte flags, int32 total_cost)
 		return cost;
 	}
 
-	if (replacement_cargo_type != CT_NO_REFIT) cost += GetRefitCost(new_engine_type); // add refit cost
+	if (replacement_cargo_type != CT_NO_REFIT) {
+		/* add refit cost */
+		int32 refit_cost = GetRefitCost(new_engine_type);
+		if (old_v->type == VEH_TRAIN && IsMultiheaded(old_v)) refit_cost += refit_cost; // pay for both ends
+		cost += refit_cost;
+	}
 
 	if (flags & DC_EXEC) {
 		new_v = GetVehicle(_new_vehicle_id);