Back-port of Volkers fix.

Back-port of Volkers fix.

    Fix a race condition in winbind leading to a crash

    When SIGCHLD handling is delayed for some reason, sending a request to a child
    can fail early because the child has died already. In this case
    async_main_request_sent() directly called the continuation function without
    properly removing the malfunctioning child process and the requests in the
    queue. The next request would then crash in the DLIST_ADD_END() in
    async_request() because the request pending for the child had been
    talloc_free()'ed and yet still was referenced in the list.

    This one is *old*...

    Volker

Jeremy.