Avoid calling wxGrid::SetCurrentCell(0, 0) when the grid has no columns or rows, as it doesn't have any cells then and doing this logically fails the precondition assert in GetColPos(). Also refactor all 6 different snippets calling SetCurrentCell() in Redimension() into a single function to simplify the code and make it more maintainable. Add a unit test verifying that this works as intended. Closes https://github.com/wxWidgets/wxWidgets/pull/1546