Winbind: Prevent cycle in children list when reaping dead child processes.

Thanks to Glenn Curtis and Kyle Stemen @ Likewise.  Their explanation is:

    In winbindd_dual.c, there is a list of children processes that
    is maintained using macros DTLIST_ADD and DTLIST_REMOVE. In the
    case when a scheduled_async_request fails, the particular child
    was located in the list, and its attributes were cleared out
    and it was reused for a subsequent async request. The bug was that
    the new request would queue the same node into the doubly-linked
    list and would result in list->next pointing to the same node as
    list itself. This would set up an infinite loop in the processing of
    the for loop when the list of children was referenced.

    Solution was to fully remove the child node from the list, such that
    it could be inserted without risk of being inserted twice.

Note that the child is re-added to the list in fork_domain_child() again.

source/winbindd/winbindd_dual.c

@@ -478,6 +478,10 @@ void winbind_child_died(pid_t pid)
 		return;
 	}
 
+	/* This will be re-added in fork_domain_child() */
+
+	DLIST_REMOVE(children, child);
+	
 	remove_fd_event(&child->event);
 	close(child->event.fd);
 	child->event.fd = 0;