Fix counts in multi-dimensional charts

Whenever the second sort field changes, we need to change
its nonblank counterpart accordingly, otherwise we'd be
updating incorrect value in the 'counts' hash. That would
lead to incorrect charting numbers being returned by
the build_reporting_chart_dim2() method.

https://bugzilla.redhat.com/show_bug.cgi?id=1169351
Merge pull request #1215 from mzazrivec/fix_chart_counts

Fix counts in multi-dimensional charts