Maximum previous checkouts displayed setting can break things if 0 or negative

This setting is used in the method open-ils.circ.copy_checkout_history.retrieve. This is used by the Show Last Few Circulations action in various places in the staff client, but unfortunately, the staff client also uses that method in a few cases when it needs to retrieve the last user who circulated a given item, and a non-positive value for the setting will break that use. For example, if an item is circulating to Patron A, and you try to check it out to Patron B, you will receive an error dialog, instead of the normal prompt offering to check the item in first.