MEDIUM: servers: Use LIST_DEL_INIT() instead of LIST_DEL().

In srv_add_to_idle_list(), use LIST_DEL_INIT instead of just LIST_DEL.
We're about to add the connection to a mt_list, and MT_LIST_ADD/MT_LIST_ADDQ
will be modified to make sure we're not adding the element if it's already
in a list.

include/proto/server.h

@@ -261,7 +261,7 @@ static inline int srv_add_to_idle_list(struct server *srv, struct connection *co
 			_HA_ATOMIC_SUB(&srv->curr_idle_conns, 1);
 			return 0;
 		}
-		LIST_DEL(&conn->list);
+		LIST_DEL_INIT(&conn->list);
 		MT_LIST_ADDQ(&srv->idle_orphan_conns[tid], (struct mt_list *)&conn->list);
 		srv->curr_idle_thr[tid]++;