Add missing index initialization in one of wxEnumProperty ctor.

In every wxEnumProperty ctor, initial index value of selected choice must be set to zero.

See #16401.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76993 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775