Avoid core dump if rpc port is in use.

The cleanup code needs to check for NULL rpcworkers thread group.

src/bitcoinrpc.cpp

@@ -881,6 +881,7 @@ void StopRPCThreads()
 
     deadlineTimers.clear();
     rpc_io_service->stop();
+    if (rpc_worker_group != NULL)
         rpc_worker_group->join_all();
     delete rpc_worker_group; rpc_worker_group = NULL;
     delete rpc_ssl_context; rpc_ssl_context = NULL;