ITS#10451 Unregister event before we close socket

We don't need the event anymore so let's make everyone's life easier by
not letting the library see different events on different sockets doing
different things on the same fd number. epoll/kqueue don't like it and
this could be a libevent bug somewhere in handling that refusal.

servers/lloadd/backend.c

@@ -72,6 +72,7 @@ done:
 
     LDAP_LIST_REMOVE( conn, next );
     if ( rc ) {
+        event_del( conn->event );
         evutil_closesocket( conn->fd );
         b->b_opening--;
         b->b_failed++;