GtkListBoxRow: add g_return_if_fail to grab_focus, don't crash
GtkListBoxRow: add g_return_if_fail to grab_focus, don't crash

Although gtk_list_box_row_grab_focus() is not a public function
it can be easily called by gtk_widget_grab_focus() with a row argument
which has been removed from the list box and has box == NULL.

https://bugzilla.gnome.org/show_bug.cgi?id=744879

ahodesuka/gtk,Lyude/gtk-,grubersjoe/adwaita,ahodesuka/gtk,Adamovskiy/gtk,grubersjoe/adwaita,alexlarsson/gtk,jadahl/gtk,davidgumberg/gtk,Adamovskiy/gtk,Lyude/gtk-,grubersjoe/adwaita,alexlarsson/gtk,jadahl/gtk,ahodesuka/gtk,Lyude/gtk-,Adamovskiy/gtk,ahodesuka/gtk,Lyude/gtk-,davidgumberg/gtk,jigpu/gtk,jigpu/gtk,chergert/gtk,davidgumberg/gtk,davidgumberg/gtk,Lyude/gtk-,Lyude/gtk-,Adamovskiy/gtk,jadahl/gtk,chergert/gtk,alexlarsson/gtk,Adamovskiy/gtk,chergert/gtk,Lyude/gtk-,grubersjoe/adwaita,jigpu/gtk,davidgumberg/gtk,ahodesuka/gtk,jadahl/gtk,grubersjoe/adwaita,grubersjoe/adwaita,davidgumberg/gtk,jigpu/gtk,chergert/gtk,jadahl/gtk,jadahl/gtk,chergert/gtk,alexlarsson/gtk,ahodesuka/gtk,ahodesuka/gtk,alexlarsson/gtk,jigpu/gtk,Adamovskiy/gtk,alexlarsson/gtk,jadahl/gtk,davidgumberg/gtk,chergert/gtk,grubersjoe/adwaita,Adamovskiy/gtk,Lyude/gtk-,ahodesuka/gtk,chergert/gtk,chergert/gtk,davidgumberg/gtk,alexlarsson/gtk,jadahl/gtk,grubersjoe/adwaita,jigpu/gtk,jigpu/gtk,jigpu/gtk,Adamovskiy/gtk,alexlarsson/gtk