Fix dragging beyond the rightmost column in generic wxHeaderCtrl

Dragging a column header in a wxHeaderCtrl and dropping it past the
rightmost column resulted in a crash due to accessing a column with
invalid index.

Fix this and correctly move the column to the last position when this
happens instead.

This bug notably affected dragging columns in wxGrid.

Closes https://github.com/wxWidgets/wxWidgets/pull/972

include/wx/generic/headerctrlg.h

@@ -98,6 +98,10 @@ private:
     // column 1 but close enough to the divider separating it from column 0)
     unsigned int FindColumnAtPoint(int x, bool *onSeparator = NULL) const;
 
+    // return the result of FindColumnAtPoint() if it is a valid column,
+    // otherwise the index of the last (rightmost) displayed column
+    unsigned int FindColumnClosestToPoint(int xPhysical) const;
+
     // return true if a drag resizing operation is currently in progress
     bool IsResizing() const;