Bug 20822: (bug 18789 follow-up) Fix HOLD_SLIP

Bug 20822: (bug 18789 follow-up) Fix HOLD_SLIP

Caused by
  commit ab47e46ea0ade643b308a8b00708d57995c57050
  Bug 18789: Pass a Koha::Patron object from returns.pl

circ/returns.pl does not send a $patron Koha::Patron object to the
template in case print_slip is set

Test plan:
Confirm a hold and print the slip

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>