Error on product already in the cart: item goes -1 qty

Hello,
when we active your plugin, we are able to place order even if the quantity is not available in stock.

To reproduce the scenario:

1. user 1 logs into the site, add an item to the cart which has stock quantity “1”
2. a second different user logs into the site and adds the same item to the cart
3. the first user concludes the order: item qty correctly updated to “0”
4. the second user concludes the order: woo commerce does not block, user is able to go to the checkout and qty is updated to “-1”.

With the plugin disabled, when the second user tries to buy the qty.0 item, an error is raised.

https://wordpress.org/plugins/woocommerce-poor-guys-swiss-knife/