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/timetable_cmd.cpp

@@ -30,6 +30,7 @@
 static void ChangeTimetable(Vehicle *v, VehicleOrderID order_number, uint16 val, ModifyTimetableFlags mtf, bool timetabled)
 {
 	Order *order = v->GetOrder(order_number);
+	assert(order != nullptr);
 	int total_delta = 0;
 	int timetable_delta = 0;
 
@@ -390,6 +391,7 @@ void UpdateVehicleTimetable(Vehicle *v, bool travelling)
 
 	if (v->cur_real_order_index >= v->GetNumOrders()) return;
 	Order *real_current_order = v->GetOrder(v->cur_real_order_index);
+	assert(real_current_order != nullptr);
 
 	VehicleOrderID first_manual_order = 0;
 	for (Order *o = v->GetFirstOrder(); o != nullptr && o->IsType(OT_IMPLICIT); o = o->next) {