Bugfix: Don’t crash anymore when moving focus to another screen (Thanks, Mirko!)

Bugfix: Don’t crash anymore when moving focus to another screen (Thanks, Mirko!)

This fixes ticket #24. The crash appeared only under special conditions:
- The screen on which you switch to must be empty or at least not as
  full as the screen on which you were before.
- You switch focus using your mouse.
- You previously selected a column/row which is not availably on the
  destination screen (for example (1,1) and switching to an empty
  screen).