WC 2.0 beta: Corrected bug if a product variation does not exist anymore

I corrected a bug that appears at least in this case:

- if you have a variable product with a few variantions
- add one of the variations to the shopping cart
- then remove this variation of the product

Consequence:
- all pages where a shopping cart appears and some others in wp-admin return the following error:

```
Fatal error: Call to a member function exists() on a non-object in /domains/xxx/public_html/wp-content/plugins/woocommerce/classes/class-wc-cart.php on line 138
```