Bug 21432: (bug 20899 follow-up) Fix patron's name display for deleted patrons

Bug 21432: (bug 20899 follow-up) Fix patron's name display for deleted patrons

Template process failed: undef error - DBIC result _type  isn't of the
_type Borrower at
/home/vagrant/kohaclone/koha-tmpl/intranet-tmpl/prog/en/modules/catalogue/issuehistory.tt
line 54.

Koha::Old::Checkout->patron should return undef if the patron record has
been removed.

Test plan:
- Check 2 items out on the same bibliographic record
- Check them in
- Delete the patron's record of one of the issuer (Is this word really
exist?)
- View the checkout history for this bib record (Home › Catalog ›
Checkout history)
=> Without this patch you get the error
=> With this patch applied one of the "Patron" cells will be empty

Signed-off-by: Claudio <costalc@gmail.com>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>