gtknotebook: Fix critical warnings on scroll events
gtknotebook: Fix critical warnings on scroll events

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=604391

Distrotech/gtk,Unity-Technologies/gtk,Distrotech/gtk,Distrotech/gtk,Distrotech/gtk,johne53/MB3Gtk-2,johne53/MB3Gtk-2,johne53/MB3Gtk-2,Distrotech/gtk,Unity-Technologies/gtk,Unity-Technologies/gtk,johne53/MB3Gtk-2,Unity-Technologies/gtk,Unity-Technologies/gtk,johne53/MB3Gtk-2