Bug 18266: Fix internal error when paying fine for lost item without.. item

Bug 18266: Fix internal error when paying fine for lost item without.. item

If a fine is created for a lost item but the itemnumber is not supplied,
the system will return it.
The item should not be mark as returned if there is no item linked to
the fine.

Test plan:
1. Turn StoreLastBorrower on
2. Create a manual invoice for a lost item, do not supply a barcode
3. Pay the fines 'Pay fines > Pay'

=> Without this patch applied you get
Can't call method "last_returned_by" on an undefined value at
/home/marc/koha/C4/Circulation.pm line 2188.

=> With this patch applied, you must not get the error.