lnrpc: fix websocket proxy data race

Was discovered in a race unit test in lightning node connect that uses
the websocket proxy to connect to the hashmail server on the client
side.
By not shadowing the err variable we end up reading and writing to the
same variable from two different goroutines, which causes the data race.

lnrpc/websocket_proxy.go

@@ -253,7 +253,7 @@ func (p *WebsocketProxy) upgradeToWebSocketProxy(w http.ResponseWriter,
 				payload = newPayload
 			}
 
-			_, err = requestForwarder.Write(payload)
+			_, err := requestForwarder.Write(payload)
 			if err != nil {
 				p.logger.Errorf("WS: error writing message "+
 					"to upstream http server: %v", err)
@@ -338,7 +338,7 @@ func (p *WebsocketProxy) upgradeToWebSocketProxy(w http.ResponseWriter,
 			continue
 		}
 
-		err = conn.WriteMessage(
+		err := conn.WriteMessage(
 			websocket.TextMessage, responseForwarder.Bytes(),
 		)
 		if err != nil {