Fix division by zero errors in QueryTable

Summary:
Code used to try to divide by zero. Fixed it to always check for the
denominator being 0 beforehand.

Original patch by Zoltan Ivanfi

BUG: 402708
FIXED-IN: 5.0.3

Reviewers: KMyMoney, tbaumgart

Reviewed By: KMyMoney, tbaumgart

Subscribers: aacid, tbaumgart, ostroffjh

Differential Revision: https://phabricator.kde.org/D17871