Fix crash on the debug console

Do not access fields in DataSourceModel::m_data that are not available

0  0x00007f3445a4bd22 in raise () at /usr/lib/libc.so.6
1  0x00007f3445a3590e in abort () at /usr/lib/libc.so.6
2  0x00007f3446362bb1 in qt_message_fatal (message=<synthetic pointer>..., context=...) at /home/apol/devel/frameworks/qt5/qtbase/src/corelib/global/qlogging.cpp:1914
3  QMessageLogger::fatal(char const*, ...) const (this=this@entry=0x7fff288a4ff8, msg=msg@entry=0x7f3446663028 "ASSERT failure in %s: \"%s\", file %s, line %d") at /home/apol/devel/frameworks/qt5/qtbase/src/corelib/global/qlogging.cpp:893
4  0x00007f344636203e in qt_assert_x(char const*, char const*, char const*, int) (where=<optimized out>, what=<optimized out>, file=<optimized out>, line=<optimized out>) at /home/apol/devel/frameworks/qt5/qtbase/src/corelib/global/qglobal.cpp:3366
5  0x00007f344973e118 in QVector<QByteArray>::at(int) const (this=0x55f42c20d338, i=7) at /home/apol/devel/kde5/include/QtCore/qvector.h:449
6  0x00007f34497305df in KWin::DataSourceModel::data(QModelIndex const&, int) const (this=0x55f42c20d320, index=..., role=6) at /home/apol/devel/frameworks/kwin/src/debug_console.cpp:1657
7  0x00007f3447674ed1 in QModelIndex::data(int) const (arole=6, this=0x7fff288a55c0) at ../../include/QtCore/../../../../../devel/frameworks/qt5/qtbase/src/corelib/itemmodels/qabstractitemmodel.h:460