Prevent negative item orders (#392)

* Pulling changes over from previous branch

* Adding exception and guard clause

src/ApplicationCore/Services/BasketService.cs

@@ -55,12 +55,13 @@ namespace Microsoft.eShopWeb.ApplicationCore.Services
             Guard.Against.Null(quantities, nameof(quantities));
             var basket = await _basketRepository.GetByIdAsync(basketId);
             Guard.Against.NullBasket(basketId, basket);
+
             foreach (var item in basket.Items)
             {
                 if (quantities.TryGetValue(item.Id.ToString(), out var quantity))
                 {
                     if (_logger != null) _logger.LogInformation($"Updating quantity of item ID:{item.Id} to {quantity}.");
-                    item.SetNewQuantity(quantity);
+                    item.SetQuantity(quantity);
                 }
             }
             basket.RemoveEmptyItems();

src/ApplicationCore/Services/OrderService.cs

@@ -33,6 +33,7 @@ namespace Microsoft.eShopWeb.ApplicationCore.Services
             var basket = await _basketRepository.FirstOrDefaultAsync(basketSpec);
 
             Guard.Against.NullBasket(basketId, basket);
+            Guard.Against.EmptyBasketOnCheckout(basket.Items);
 
             var catalogItemsSpecification = new CatalogItemsSpecification(basket.Items.Select(item => item.CatalogItemId).ToArray());
             var catalogItems = await _itemRepository.ListAsync(catalogItemsSpecification);