use default if request port is 0.

lichao

2021-06-01 7c8e2433992aac41bdeb0d7d0861b738d9866628

 src/exported_symbols

@@ -5,6 +5,7 @@
   BHUnregister;
   BHRegisterTopics;
   BHQueryTopicAddress;
   BHQueryProcs;
   BHSubscribeTopics;
   BHStartWorker;
   BHHeartbeatEasy;