fix: diag_timer_cb UAF — обнуляем rc->diag_timer до раннего возврата

src/proxy/tcp_proxy_server.c

@@ -156,6 +156,7 @@ static void on_error_cb(struct tcp_conn* tc, int err, void* arg) {
 
 static void diag_timer_cb(void* arg) {
     struct tcp_proxy_server_conn* rc = (struct tcp_proxy_server_conn*)arg;
+    rc->diag_timer = NULL;
     struct tcp_conn* tc = rc->tc;
     if (!tc || tc->sock == SOCKET_INVALID) return;