gtkentrycompletion: check return value because It can be non-UTF8
gtkentrycompletion: check return value because It can be non-UTF8

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=624270

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