Fix a race condition in winbind leading to a crash

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