Bug 18146: (follow-up) Remove warn for uninitialized value

Bug 18146: (follow-up) Remove warn for uninitialized value

In the previous patch you may have noticed many warns when running the tests

We add guarantor charges to a variable to determine if over the limit, but we don't initialize that value

We should

To test:
1 - Apply first patch and follow test plan
2 - Note warns when proving test
3 - Apply this patch
4 - prove
5 - No more warns

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>