gtkprintbackendcups: Fix a crash in gtk_print_backend_cups_finalize()
gtkprintbackendcups: Fix a crash in gtk_print_backend_cups_finalize()

If GtkPrintBackendCups is finalized and cups_get_printer_list hasn't
been called, g_object_unref is called for the GDBusConnection pointer
that is NULL. Use g_clear_object() instead.

https://bugzilla.gnome.org/show_bug.cgi?id=696546

Lyude/gtk-,chergert/gtk,Distrotech/gtk2,davidgumberg/gtk,Lyude/gtk-,grubersjoe/adwaita,jessevdk/gtk,bratsche/gtk-,ahodesuka/gtk,grubersjoe/adwaita,davidgumberg/gtk,alexlarsson/gtk,alexlarsson/gtk,ahodesuka/gtk,grubersjoe/adwaita,jigpu/gtk,jadahl/gtk,davidgumberg/gtk,grubersjoe/adwaita,jadahl/gtk,ebassi/gtk,jadahl/gtk,Adamovskiy/gtk,ahodesuka/gtk,davidgumberg/gtk,Adamovskiy/gtk,ebassi/gtk,jessevdk/gtk,grubersjoe/adwaita,alexlarsson/gtk,jessevdk/gtk,Sidnioulz/SandboxGtk,alexlarsson/gtk,Adamovskiy/gtk,grubersjoe/adwaita,ebassi/gtk,Adamovskiy/gtk,Lyude/gtk-,jigpu/gtk,Distrotech/gtk2,chergert/gtk,davidgumberg/gtk,jigpu/gtk,jigpu/gtk,Sidnioulz/SandboxGtk,bratsche/gtk-,chergert/gtk,jadahl/gtk,chergert/gtk,davidgumberg/gtk,chergert/gtk,msteinert/gtk,msteinert/gtk,jadahl/gtk,jigpu/gtk,alexlarsson/gtk,grubersjoe/adwaita,msteinert/gtk,ebassi/gtk,Lyude/gtk-,Adamovskiy/gtk,Lyude/gtk-,ebassi/gtk,davidgumberg/gtk,ebassi/gtk,jadahl/gtk,msteinert/gtk,Sidnioulz/SandboxGtk,grubersjoe/adwaita,jessevdk/gtk,jadahl/gtk,msteinert/gtk,ahodesuka/gtk,jigpu/gtk,ahodesuka/gtk,chergert/gtk,chergert/gtk,msteinert/gtk,Distrotech/gtk2,jessevdk/gtk,jessevdk/gtk,Distrotech/gtk2,Sidnioulz/SandboxGtk,bratsche/gtk-,ahodesuka/gtk,jigpu/gtk,bratsche/gtk-,alexlarsson/gtk,ahodesuka/gtk,bratsche/gtk-,Adamovskiy/gtk,jigpu/gtk,Sidnioulz/SandboxGtk,bratsche/gtk-,Lyude/gtk-,Distrotech/gtk2,Adamovskiy/gtk,Distrotech/gtk2,chergert/gtk,jessevdk/gtk,Sidnioulz/SandboxGtk,Lyude/gtk-,davidgumberg/gtk,ahodesuka/gtk,alexlarsson/gtk,Adamovskiy/gtk,jadahl/gtk,Lyude/gtk-,alexlarsson/gtk