Now that the in_use flag is set for potions being quaffed, use the in_use
flag in general in destroy_item to avoid destroying the in use object, on
the assumption that the caller will call useup when finished. There are a
few places that set then unset in_use, but these don't currently result in
a call to destroy_item. The current_wand hack was not removed, since its
logic appears to allow destroy_item to still destroy the item.
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