Bug 21168: Handle deleted checked-in items on checking in

Bug 21168: Handle deleted checked-in items on checking in

circ/returns.pl generates a server error when attempting to checkin an item while
the list of previously-returned items contains an item that has been deleted.
Error text: Can't call method "biblio" on an undefined value at /usr/share/koha/intranet/cgi-bin/circ/returns.pl line 606.

Test plan:
- checkout two items
- checkin item1
- on a separate tab, delete item1
- on the original tab (which still shows item1 in the list of Checked-In Items), attempt to check in item2
=> No error appears

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>