LP#1749795 Close circ on adjustment when rebilled

LP#1749795 Close circ on adjustment when rebilled

When doing a re-bill, we were aborting too early when it rebilled to
exactly zero.  Move the 'zero' check to the top of the loop, and
thereby make sure we check if the xact can be closed if we do any
adjustment, up or down.

To test:
- Run new billing test from previous commit, see it fail
- Apply patch
- Run tests again; all should pass

Signed-off-by: Dan Wells <dbw2@calvin.edu>
Signed-off-by: Jeff Davis <jdavis@sitka.bclibraries.ca>
Signed-off-by: Chris Sharp <csharp@georgialibraries.org>