Bug 19487: Do not return an item if not checked out

Bug 19487: Do not return an item if not checked out

To recreate:
1 - Manually add a lost fine to a ptron and include a barcode
2 - Attempt to write off the fine
3 - Internal server error
4 - Checkout an item and mark lost to checkin and fine
5 - Attempt to write off line
6 - Internal server error

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>