Ensure (Get|Set)ColumnWidth is not called on a non existant column.

The AutomaticColumnWidth method can be called when the game list does
not have any columns (for example when dolphin is starting).  In this
case wxWidgets assertions fail when (Get|Set)ColumnWidth are called on
columns that do not exist.  So check to make sure there are columns to
prevent this.
Merge pull request #518 from glennricster/master

Ensure (Get|Set)ColumnWidth is not called on a non existant column.