Bug 22891: (bug 21206 follow-up) Fix RenewHold (ILS-DI)

Bug 22891: (bug 21206 follow-up) Fix RenewHold (ILS-DI)

From
  commit 31c29fd31f557306233e6a2936148a5bb10b89a1
  Bug 21206: Replace C4::Items::GetItem

A missing 'unless' test made RenewHold return prematurely:
  return $issue;
should have been
  return unless $issue;

Test plan:
Hit /cgi-bin/koha/ilsdi.pl?service=RenewLoan&patron_id=51&item_id=756

With different situations:
1. Item is checked out to the patron
2. Item is not checked out to the patron
3. Item does not exist
4. Patron does not exist

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>