Fix assert with setting current cell in wxGrid::Redimension()

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

include/wx/generic/grid.h

@@ -2309,6 +2309,10 @@ private:
     // common part of Clip{Horz,Vert}GridLines
     void DoClipGridLines(bool& var, bool clip);
 
+    // Redimension() helper: update m_currentCellCoords if necessary after a
+    // grid size change
+    void UpdateCurrentCellOnRedim();
+
     // update the sorting indicator shown in the specified column (whose index
     // must be valid)
     //