Don't cache cells being measured in wxHtmlListBox.

This can result in a crash if the measuring code is called, possibly
indirectly, from a method of a cell object itself and if that cell is
displaced from the cache while caching the cell created in OnMeasureItem().

Closes #16651.

include/wx/htmllbox.h

@@ -148,6 +148,9 @@ private:
     // returns index of item that contains given HTML cell
     size_t GetItemForCell(const wxHtmlCell *cell) const;
 
+    // Create the cell for the given item, caller is responsible for freeing it.
+    wxHtmlCell* CreateCellForItem(size_t n) const;
+
     // return physical coordinates of root wxHtmlCell of n-th item
     wxPoint GetRootCellCoords(size_t n) const;