ws-chat-server: avoid SIGSEGV in case of evws_new_session() failure

sample/ws-chat-server.c

@@ -152,13 +152,19 @@ on_ws(struct evhttp_request *req, void *arg)
 	socklen_t len;
 
 	client = calloc(sizeof(*client), 1);
+	addr2str((struct sockaddr *)&addr, client->name, sizeof(client->name));
 	client->evws = evws_new_session(req, on_msg_cb, client, 0);
+	if (!client->evws) {
+		free(client);
+		log_d("Failed to create session (for %s)\n", client->name);
+		return;
+	}
+
 	fd = bufferevent_getfd(evws_connection_get_bufferevent(client->evws));
 
 	len = sizeof(addr);
 	getpeername(fd, (struct sockaddr *)&addr, &len);
 
-	addr2str((struct sockaddr *)&addr, client->name, sizeof(client->name));
 	log_d("New client joined from %s\n", client->name);
 
 	evws_connection_set_closecb(client->evws, on_close_cb, client);