FindFirstFreeSlotThatFitsItem was returning 0 instead of INVALID_INDEX
when no free slot was found. This caused items to be placed in slot 0
(charm equipment slot) when inventory was full, overwriting equipped items.
Changes:
- Fix FindFirstFreeSlotThatFitsItem to return INVALID_INDEX when no slot found
- Add defensive check in PutItemInInventoryWithStacking to protect equipment
slots 0-22 from being targeted for item placement
