Quatity add-on allows overbooking in backend

[Valentina R]

Check the attached video to see how to easily reproduce it.

This is not possible on the frontend because on the frontend it selects the next available slot. But in the backend it works differently and doesn’t show the next available slot, thus allowing overbookings if not careful.

Reported here.

[Željan Topić]

Fixed with 4.9.8