fix bug in rpc_client.go (#510)

Co-authored-by: guyinyou <[email protected]>

golang/rpc_client.go

@@ -102,7 +102,7 @@ func (rc *rpcClient) GracefulStop() error {
 	rc.mux.Lock()
 	sugarBaseLogger.Warnf("close rpc client, target=%s", rc.target)
 	closeResult := rc.conn.Close()
-	rc.mux.Lock()
+	rc.mux.Unlock()
 	return closeResult
 }