[gdm] settings: Make sure to set old settings global to NULL on finalize

[Ray Strode <halfline src gnome org>]

commit b3aabada88350bfa10c97e6d5a8861ec4ac0a08d
Author: Sam Spilsbury <smspillaz gmail com>
Date:   Tue Sep 5 14:02:08 2017 -0400

    settings: Make sure to set old settings global to NULL on finalize
    
    https://bugzilla.gnome.org/show_bug.cgi?id=787288

 common/gdm-settings.c |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)
---
diff --git a/common/gdm-settings.c b/common/gdm-settings.c
index 1040fbb..88b079b 100644
--- a/common/gdm-settings.c
+++ b/common/gdm-settings.c
@@ -187,6 +187,8 @@ gdm_settings_finalize (GObject *object)
                 g_object_unref (settings->priv->backend);
         }
 
+        settings_object = NULL;
+
         G_OBJECT_CLASS (gdm_settings_parent_class)->finalize (object);
 }