[calendar] Check out of bounds array access in QLocale lookup

Summary:
If we have a broken locale setup we don't have any uiLanugages to look
up the relevant locale object for.

In that case use the system locale.

Test Plan:
Had a crash here

#11 0x00007ff982aab0b2 in QList<QString>::at(int) const (this=0x7ffd50b1d928, i=0) at /opt/qt5/include/QtCore/qlist.h:571
#12 0x00007ff982aaaa4a in Calendar::monthName() const (this=0x5640cce026f0) at /home/david/projects/kde5/src/frameworks/plasma-framework/src/declarativeimports/calendar/calendar.cpp:202
#13 0x00007ff982aa14e9 in Calendar::qt_static_metacall(QObject*, QMetaObject::Call, int, void**) (_o=0x5640cce026f0, _c=QMetaObject::ReadProperty, _id=9, _a=0x7ffd50b1dd40) at src/declarativeimports/calendar/calendarplugin_autogen/EWIEGA46WW/moc_calendar.cpp:340

Reviewers: Plasma, vkrause

Reviewed By: vkrause

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D25960