gio: avoid g_object_ref(NULL) in g_dbus_object_manager_client_finalize
gio: avoid g_object_ref(NULL) in g_dbus_object_manager_client_finalize

When establishing a D-Bus connection failed, g_dbus_object_manager_client_finalize()
calls g_object_ref(manager->priv->connection) when that pointer is NULL,
which is considered and logged as error by glib.

https://bugzilla.gnome.org/show_bug.cgi?id=732984

MathieuDuponchelle/glib,endlessm/glib,mzabaluev/glib,MathieuDuponchelle/glib,cention-sany/glib,tamaskenez/glib,mzabaluev/glib,endlessm/glib,tchakabam/glib,cention-sany/glib,cention-sany/glib,mzabaluev/glib,endlessm/glib,krichter722/glib,ieei/glib,mzabaluev/glib,lukasz-skalski/glib,tamaskenez/glib,johne53/MB3Glib,Distrotech/glib,gale320/glib,mzabaluev/glib,krichter722/glib,ieei/glib,gale320/glib,tamaskenez/glib,ieei/glib,MathieuDuponchelle/glib,tamaskenez/glib,tchakabam/glib,tamaskenez/glib,johne53/MB3Glib,tchakabam/glib,MathieuDuponchelle/glib,Distrotech/glib,krichter722/glib,cention-sany/glib,MathieuDuponchelle/glib,Distrotech/glib,johne53/MB3Glib,gale320/glib,krichter722/glib,lukasz-skalski/glib,johne53/MB3Glib,johne53/MB3Glib,cention-sany/glib,krichter722/glib,gale320/glib,ieei/glib,lukasz-skalski/glib,tchakabam/glib,endlessm/glib,lukasz-skalski/glib,Distrotech/glib,gale320/glib,lukasz-skalski/glib,Distrotech/glib,endlessm/glib,ieei/glib,tchakabam/glib,johne53/MB3Glib