Bug 20120: Prevent writeoffs of more than the amount owed for a fee

Bug 20120: Prevent writeoffs of more than the amount owed for a fee

In short, it is possible to 'over-writeoff' a fee such that the value of the fee is now a credit.

Test Plan:
1) Attempt to writeoff a single fee, set the amount input to more than
   the amount of the fee.
2) Note the writeoff makes the fee go negative.
3) Apply this patch
4) Attempt to repeat step 1, you should be unable to write-off an
   amount more than the amount outstanding for the fee!

Signed-off-by: George Williams <george@nekls.org>
Signed-off-by: George Williams <george@nekls.org>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>