Fix wxOwnerDrawnComboBox keyboard navigation with duplicate items.

Fix wxOwnerDrawnComboBox keyboard navigation with duplicate items.

If the combobox contained duplicate strings (i.e. multiple items with the
same string value, but differing indexes), then navigating to the second
and subsequent ones skipped to the first occurence instead. We need to
preserve the index.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65463 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775