Add Graceful Assert for KoToolBox::buttonSize Display Indexing

Something about my specific development environment was causing a crash due
to QGuiApplication::screens() indexing throwing an Out of Range error.
It likely has something to do with my vertical-right oriented display I
just set up. I'll do more investigation of this issue, and whether its a
QT or GNOME bug. For now, preventing a hard crash for simple GUI
button size scaling is probably appropriate.