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