Fixed intermittent crash in treelist when deleting a selected item.

On Win7 64-bit, I have been experiencing crashes deleting selected items from a Treelist control. It appears that the source of the crash was the possibility that the tl->scrolled_item could still be referencing the item that we've deleted, leading to some oddities during the subsequent updates after the deletion occurs. Additionally, I think that there may have been a dangling reference as the selected item as well because, if no selected item existed, it could cause our deleted item to hang around as the selected item. I could be very wrong on the selected item stuff, though.