Fix wxPGProperty::InsertChoice

Take into account that wxPGProperty::GetChoiceSelection can return either 0 (for wxEnumProperty) or -1 (for other properties) if choice value list is empty and initialize new selection index properly.

See #16401.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@77012 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775