Update old selections after clearing wxListBox in wxGTK.

Don't keep stale selected item indices in m_oldSelections after clearing the
listbox, call UpdateOldSelections() from DoClear() in wxGTK just as we do in
the other ports.

Closes #14359.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71626 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/gtk/listbox.cpp

@@ -497,6 +497,8 @@ void wxListBox::DoClear()
     gtk_list_store_clear( m_liststore ); /* well, THAT was easy :) */
 
     GTKEnableEvents();
+
+    UpdateOldSelections();
 }
 
 void wxListBox::DoDeleteOneItem(unsigned int n)