Bug 19205 - Pay selected fine generates 500 error

Bug 19205 - Pay selected fine generates 500 error

Using the pay selected option from the borrowers account, to pay for one specific fine among other gives a 500 error, despite the payment going through.

Test Plan:
1) Add two fines using the Create manual invoice function.
2) Select one fine and "pay selected".
3) Pay a part of the amount.
4) Note error
5) Apply this patch
6) Repeat steps 1-3
7) No error!

Signed-off-by: Andreas Hedström Mace <andreas.hedstrom.mace@sub.su.se>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>