Codechange: add non-nullptr asserts in cases where it should never be nullptr

Though where similar calls are checked for nullptr as in those instances of
the use of that function it can actually return nullptr. In other words, write
down the assumption that the function never returns nullptr in an assert.

src/script/api/script_order.cpp

@@ -68,6 +68,7 @@ static const Order *ResolveOrder(VehicleID vehicle_id, ScriptOrder::OrderPositio
 		if (order_position == ScriptOrder::ORDER_INVALID) return nullptr;
 	}
 	const Order *order = v->GetFirstOrder();
+	assert(order != nullptr);
 	while (order->GetType() == OT_IMPLICIT) order = order->next;
 	while (order_position > 0) {
 		order_position = (ScriptOrder::OrderPosition)(order_position - 1);
@@ -92,6 +93,7 @@ static int ScriptOrderPositionToRealOrderPosition(VehicleID vehicle_id, ScriptOr
 
 	int res = (int)order_position;
 	const Order *order = v->orders->GetFirstOrder();
+	assert(order != nullptr);
 	for (; order->GetType() == OT_IMPLICIT; order = order->next) res++;
 	while (order_position > 0) {
 		order_position = (ScriptOrder::OrderPosition)(order_position - 1);
@@ -132,6 +134,7 @@ static int ScriptOrderPositionToRealOrderPosition(VehicleID vehicle_id, ScriptOr
 	if (!IsValidVehicleOrder(vehicle_id, order_position)) return false;
 
 	const Order *order = ::Vehicle::Get(vehicle_id)->GetOrder(ScriptOrderPositionToRealOrderPosition(vehicle_id, order_position));
+	assert(order != nullptr);
 	return order->GetType() == OT_CONDITIONAL;
 }
 
@@ -141,6 +144,7 @@ static int ScriptOrderPositionToRealOrderPosition(VehicleID vehicle_id, ScriptOr
 	if (!IsValidVehicleOrder(vehicle_id, order_position)) return false;
 
 	const Order *order = ::ResolveOrder(vehicle_id, order_position);
+	assert(order != nullptr);
 	return order->GetType() == OT_DUMMY;
 }
 
@@ -172,6 +176,7 @@ static int ScriptOrderPositionToRealOrderPosition(VehicleID vehicle_id, ScriptOr
 	if (order_position == ORDER_CURRENT) {
 		int cur_order_pos = ::Vehicle::Get(vehicle_id)->cur_real_order_index;
 		const Order *order = ::Vehicle::Get(vehicle_id)->GetFirstOrder();
+		assert(order != nullptr);
 		int num_implicit_orders = 0;
 		for (int i = 0; i < cur_order_pos; i++) {
 			if (order->GetType() == OT_IMPLICIT) num_implicit_orders++;