Avoid assertion failure when GtkTreeView interactive search is started.
The treeview does an unselect_all causing a "changed" signal with no selection.
The problem is easily triggered by pressing Ctrl-F (the "start-interactive-search" key binding)


git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76361 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775