We used to fall Item.filterData() for this but that method is meant for
converting item data to block data during placement and does the wrong
thing for this case. Instead we just see if the item should have data and
if not set it to zero. We also have to filter wool data explicitly because
clients crash when given invalid wool data.
The player would have no permissions (other than their OP status)
when checked in the Quit event. This is because we removed permissions
before the event occurred. By calling it afterwards, we can persist
the data until the server finally removes the player.
