Ensure not to overflow buf in udp server example

PUBLISHED_FROM=413f5073b0fc267d66ce04d8d40e9e9d7abac94d

examples/mcast_udp_server/mcast_udp_server.c

@@ -79,7 +79,7 @@ int main(int argc, char **argv) {
   mg_mgr_init(&mgr, NULL);
   {
     char listen[256];
-    sprintf(listen, "udp://%s", port);
+    snprintf(listen, sizeof(listen), "udp://%s", port);
     nc = mg_bind(&mgr, listen, ev_handler);
   }