Update payment_total on void.

Currently an orders payment total is not updated when a payment is
voided. This leads to a variety of things which are not correct. If $100
is paid on an order, then that payment is voided, it will reset the
payment_total to $0, rather than leaving it at $100.

Fixes #5822
Merge pull request #22 from gmacdougall/update-payment-total-on-void-2-3-dev

Update payment_total on void.