Check that item ID is initialized when inserting it in wxListCtrl.

Check that item ID is initialized when inserting it in wxListCtrl.

Using uninitialized ID happened to work by accident with 2.8 but it doesn't
work any more, try to detect this as early as possible and give a clear
explanation of why it doesn't to help fixing the broken code that used to
accidentally work.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71381 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775