Sometimes when adding items from buffer to inventory, proliferator point is missed by 1

**Describe the bug**
Sometimes when adding items from buffer to player inventory, the proliferator point of added items will be missed by 1,
causing 1 item in the inventory to have lv2 proliferator, and 1 extra proliferator point in the buffer.

Fix is simple and I will submit a PR shortly.

**Screenshots**
(I was going to do this but DSP updated just now so I can't..)
