Fix "ghost stacks" created when a player clicks an item on the ground:

Ekdohibs · Ekdohibs · commit 936c6f577ad7 · 2014-03-22T18:15:33.000+01:00
since the object is not immediately removed, any other code may still
think an object is there, therefore leading to item duplication. This code
therefore sets the itemstring to '' after the object is picked up to avoid
such issues
diff --git a/builtin/item_entity.lua b/builtin/item_entity.lua
@@ -116,6 +116,7 @@ minetest.register_entity("__builtin:item", {
 				return
 			end
 		end
+		self.itemstring = ''
 		self.object:remove()
 	end,
 })
