Bug 16378: Overdues.pm: Can't call method "store" without a package or object reference

Bug 16378: Overdues.pm: Can't call method "store" without a package or object reference

Overdues.pm line 588 tries to do an $accountline->set with column outstanding
instead of amountoutstanding.
This patch fixes it.

To test:
- Verify that code change is correct.
- Try to trigger a situation where an existing fine is updated
  (see comment in line 574 in C4/Overdues.pm)

Signed-off-by: Jacek Ablewicz <abl@biblos.pk.edu.pl>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Brendan Gallagher <brendan@bywatersolutions.com>