Do not crash when encountering invalid plugin info.

I've recently seen crashes when Plasma::RunnerManager calls
KPluginInfo::fromServices which then called setConfig on an
invalid KPluginInfo object which triggered the internal
KPLUGININFO_ISVALID_ASSERTION which uses a qFatal to kill the
whole parent application, even in release mode.

Differential Revision: https://phabricator.kde.org/D338