Fix a number of issues found by the OpenSuse security team in networkhelper

Summary:
See https://bugzilla.opensuse.org/show_bug.cgi?id=1151190 and its dependencies
for details.

Test Plan: Helper still collects data correctly.

Reviewers: Plasma, maltek

Reviewed By: maltek

Subscribers: maltek, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D24359