task: avoid context lock when setting source name

If you set the source name after attaching to the context, you have to
lock the context to free/assign the new source name.

https://bugzilla.gnome.org/show_bug.cgi?id=765861

gio/gtask.c

@@ -1170,8 +1170,8 @@ g_task_return (GTask           *task,
 
   /* Otherwise, complete in the next iteration */
   source = g_idle_source_new ();
-  g_task_attach_source (task, source, complete_in_idle_cb);
   g_source_set_name (source, "[gio] complete_in_idle_cb");
+  g_task_attach_source (task, source, complete_in_idle_cb);
   g_source_unref (source);
 }