Don't crash on malformed syntax highlighting files
Test Plan:
- Create malformed syntax highlighting file with missing end tags
- Place it in ~/.local/share/katepart5/syntax/
- Open Kate with file which uses that syntax highlighting
Reviewers: cullmann, dhaumann
Reviewed By: cullmann
Subscribers: kwrite-devel, kde-frameworks-devel
Tags: Kate, Frameworks
Differential Revision: https://phabricator.kde.org/D19533