backend: fix double free of priv->font_name

If the backend was disposed then priv->font_name would be freed but not
set to NULL and so if clutter_backend_get_font_name was then called it
would double free priv->font_name.

clutter/clutter-backend.c

@@ -97,6 +97,7 @@ clutter_backend_dispose (GObject *gobject)
     }
 
   g_free (backend->priv->font_name);
+  backend->priv->font_name = NULL;
 
   clutter_backend_set_font_options (backend, NULL);