Don't crash when there is no selected computer engine.

You can reproduce this crash doing this:
1. Choose in the new game dialog "Computer engine" for player 1 or player 2.
2. Make sure there is no selected item in the combobox; you can do this either
- just not touching that combobox if it's the very first you are running the
program.
- clicking on "Configure Engines" and removing all the selected engines;
thus the combobox will be empty.
3. Click the OK of the new game dialog.