check null pointer

tutorials/udp/mdns-server/main.c

@@ -12,6 +12,7 @@ int main(void) {
 
   // Desired name must NOT have any dots in it, nor a domain
   c = mg_mdns_listen(&mgr, "Mongoose");  // Start mDNS server
+  if (c == NULL) return 1;
   // if not using our built-in TCP/IP stack, pass the IP address you want to
   // use as a response, this depends on your underlying TCP/IP stack and number
   // of interfaces available