goutputstream: Fix missing call to clear_pending in flush_async

If flush_async is deleted by a child class, then calling
g_output_stream_flush_async would leave the GOutputStream in an invalid
state. I'm not aware of any GOutputStream that would be affected by this
issue, but might as well fix it.

https://bugzilla.gnome.org/show_bug.cgi?id=738277

gio/goutputstream.c

@@ -1340,6 +1340,7 @@ g_output_stream_flush_async (GOutputStream       *stream,
   
   if (class->flush_async == NULL)
     {
+      g_output_stream_clear_pending (stream);
       g_task_return_boolean (task, TRUE);
       g_object_unref (task);
       return;