kcm_fonts: Initialize variables properly
Summary: Currently they have undefined values, which leads to garbage in fonts.conf.
Test Plan:
Did some changes, no <double>nan</double> or <double>-2.14748e+09</double>
anymore.
Reviewers: Plasma, bshah
Reviewed By: Plasma, bshah
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D21641