void silc_async_free(SilcAsyncOperation op);

    Stop the asynchronous operation.  If the asynchronous operation ended
    normally (ie. it was not aborted) this function must be called by the
    caller who called silc_async_alloc.  The `op' will become invalid after
    this and the upper layer must not call silc_async_abort after this
    function is called.  The layer that calls this, must call some other
    completion callback to the upper layer, so that it knows that the
    asynchronous operation is completed.