Comment 7 for bug 199145

This bug comes from a buggy .desktop files, like /usr/share/applications/Swiftfox.desktop which should not contains a MimeType field
The assert has been turned into a warning in trunk, see attached patch backported from r781958.

Workaround:
- use "sudo update-desktop-database -v 2>&1|grep invalid" to find which .desktop files make kbuildsycoca4 crashes
- remove it or correct it