[gnome-control-center] thunderbolt: Fix GCancellable leak

[Robert Ancell <rancell src gnome org>]

commit 42c8e63b1e9c13a42e819ce20db315d33bb15f1e
Author: Robert Ancell <robert ancell canonical com>
Date:   Fri Jan 31 10:06:06 2020 +1300

    thunderbolt: Fix GCancellable leak

 panels/thunderbolt/cc-bolt-panel.c | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/panels/thunderbolt/cc-bolt-panel.c b/panels/thunderbolt/cc-bolt-panel.c
index 81a875b03..805666ed6 100644
--- a/panels/thunderbolt/cc-bolt-panel.c
+++ b/panels/thunderbolt/cc-bolt-panel.c
@@ -900,6 +900,7 @@ cc_bolt_panel_dispose (GObject *object)
 
   /* cancel any ongoing operation */
   g_cancellable_cancel (panel->cancel);
+  g_clear_object (&panel->cancel);
 
   /* Must be destroyed in dispose, not finalize. */
   g_clear_pointer ((GtkWidget **) &panel->device_dialog, gtk_widget_destroy);