Bug 24146: Corrections to UpdateFine logic concerning maxFine

Bug 24146: Corrections to UpdateFine logic concerning maxFine

The `maxFine` system preference actually refers to the amount of
outsanding debt (in fines) a patron may have at a given time. This patch
corrects the functionality of UpdateFine such that it properly respects
that counter given the payment of accruing fines scenario.

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>