MINOR: connection: improve list api usage

Replace !LIST_ISEMPTY by LIST_ADDED and LIST_DEL+LIST_INIT by
LIST_DEL_INIT for connection session list.
diff --git a/include/haproxy/connection.h b/include/haproxy/connection.h
index 2e01a2d..37d3f42 100644
--- a/include/haproxy/connection.h
+++ b/include/haproxy/connection.h
@@ -468,7 +468,7 @@
 		/* The connection is private, so remove it from the session's
 		 * connections list, if any.
 		 */
-		if (!LIST_ISEMPTY(&conn->session_list))
+		if (LIST_ADDED(&conn->session_list))
 			session_unown_conn(conn->owner, conn);
 	}
 	else {
diff --git a/include/haproxy/session.h b/include/haproxy/session.h
index 05c56f6..59945b2 100644
--- a/include/haproxy/session.h
+++ b/include/haproxy/session.h
@@ -80,8 +80,7 @@
 
 	if (conn->flags & CO_FL_SESS_IDLE)
 		sess->idle_conns--;
-	LIST_DEL(&conn->session_list);
-	LIST_INIT(&conn->session_list);
+	LIST_DEL_INIT(&conn->session_list);
 	list_for_each_entry(srv_list, &sess->srv_list, srv_list) {
 		if (srv_list->target == conn->target) {
 			if (LIST_ISEMPTY(&srv_list->conn_list)) {