Reset wxAuiNotebook selection to wxNOT_FOUND if it becomes empty.

Reset wxAuiNotebook selection to wxNOT_FOUND if it becomes empty.

The selection was 0 in a notebook without any pages which was wrong as it must
always be less than the page count.

Fix this in wxAuiNotebook itself and add an assert checking for this invariant
to the sample.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63642 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775