Fix Vehicle::CopyVehicleConfigAndStatistics not releasing unit number

See: https://github.com/OpenTTD/OpenTTD/issues/12388

src/train_cmd.cpp

@@ -2225,8 +2225,7 @@ CommandCost CmdMoveRailVehicle(TileIndex tile, DoCommandFlag flags, uint32_t p1,
 			}
 			/* Remove stuff not valid anymore for non-front engines. */
 			DeleteVehicleOrders(src);
-			Company::Get(src->owner)->freeunits[src->type].ReleaseID(src->unitnumber);
-			src->unitnumber = 0;
+			src->ReleaseUnitNumber();
 			if (!_settings_game.vehicle.non_leading_engines_keep_name) {
 				src->name.clear();
 			}

src/vehicle.cpp

@@ -3710,6 +3710,17 @@ void Vehicle::ResetRefitCaps()
 	for (Vehicle *v = this; v != nullptr; v = v->Next()) v->refit_cap = v->cargo_cap;
 }
 
+/**
+ * Release the vehicle's unit number.
+ */
+void Vehicle::ReleaseUnitNumber()
+{
+	if (this->unitnumber != 0) {
+		Company::Get(this->owner)->freeunits[this->type].ReleaseID(this->unitnumber);
+		this->unitnumber = 0;
+	}
+}
+
 static bool ShouldVehicleContinueWaiting(Vehicle *v)
 {
 	if (v->GetNumOrders() < 1) return false;

src/vehicle_base.h

@@ -907,6 +907,8 @@ public:
 
 	void ResetRefitCaps();
 
+	void ReleaseUnitNumber();
+
 	/**
 	 * Copy certain configurations and statistics of a vehicle after successful autoreplace/renew
 	 * The function shall copy everything that cannot be copied by a command (like orders / group etc),
@@ -917,6 +919,7 @@ public:
 	{
 		this->CopyConsistPropertiesFrom(src);
 
+		this->ReleaseUnitNumber();
 		this->unitnumber = src->unitnumber;
 
 		this->current_order = src->current_order;