Fix UDP buffer memory leak when max clients reached

Caught by Coverity-scan CID 330055

src/sbbs3/services.c

@@ -2247,6 +2247,7 @@ void DLLCALL services_thread(void* arg)
 							,service[i].protocol, host_ip, inet_addrport(&client_addr));
 
 					if(service[i].max_clients && protected_uint32_value(service[i].clients) + 1 > service[i].max_clients) {
+						FREE_AND_NULL(udp_buf);
 						lprintf(LOG_WARNING,"%04d %s !MAXIMUM CLIENTS (%u) reached, access denied"
 							,client_socket, service[i].protocol, service[i].max_clients);
 						close_socket(client_socket);