Child products - more than in stock can be ordered

[olgri]

VirtueMart 2.6.6
Stock control and custom fields with stockable variant are used.

1. Set "In Stock" to 13 for a child product.
2. Add 10 (child) products to cart. The shopping cart now shows "Product - child product x 10" - one line.
3. Go to the product page and add 5 more. Now you can see two lines in the Shopping Cart: "Product - child product x 10" and "Product - child product x 5".
4. Proceed with checkout and now you have successfully purchased 15 items (even though there are only 13 in stock).

When you add a product to cart for the second time, it should update the current product in the cart instead of adding it as a different item.
This works fine on normal (simple) products but not with stockable variant 

Is there a patch for this bug??