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