Bug 28053: Squash warning in C4::Members

Bug 28053: Squash warning in C4::Members

This patch sets guarantess_non_issues_charges to '0' upon creation of
the variable and thus prevents 'Use of uninitialized value
$guarantees_non_issues_charges in numeric gt (>) at
/home/martin/kohaclone/C4/Members.pm line 183.' being thrown when the
patron has guaruntees without any charges.

Test plan
1. Run t/db_dependant/SIP/SIPServer.t and note the warning above is
   thrown at least once.
2. Apply the patch
3. Run the test again and note the warning has gone.

Signed-off-by: Petro Vashchuk <stalkernoid@gmail.com>
Signed-off-by: Joonas Kylmälä <joonas.kylmala@helsinki.fi>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>