Bug 20323: Fix batch patron modification tool

Bug 20323: Fix batch patron modification tool

Test plan:
1) Go to Tools -> Batch patrons modification
2) Select some patrons and try to change any field
--> without patch - it exploads with log:
Can't call method "category" on unblessed reference at /home/vagrant/kohaclone/tools/modborrowers.pl
--> with patch - the modification is properly made

Signed-off-by: Roch D'Amour <roch.damour@inlibro.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>