Fix blank wxComboBox appearance broken by the changes of r75134.

We didn't repaint the owner drawn combobox items any more because they were
not found by MSWFindItem().

Fix this by restricting the check to the real windows with the same ID as the
combobox, not the dropdown items which are not windows at all.

See #15674.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75196 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775