rpcclient: correctly handle request channel closure

wsReader() closes c.done first and then goes over the list of
c.respChannels. Technically this means that any of the two can be taken in
this select.

Signed-off-by: Roman Khimov <roman@nspcc.ru>

pkg/rpcclient/wsclient.go

@@ -638,7 +638,10 @@ func (c *WSClient) makeWsRequest(r *neorpc.Request) (*neorpc.Response, error) {
 	select {
 	case <-c.done:
 		return nil, errors.New("connection lost while waiting for the response")
-	case resp := <-ch:
+	case resp, ok := <-ch:
+		if !ok {
+			return nil, errors.New("connection lost while waiting for the response")
+		}
 		c.unregisterRespChannel(r.ID)
 		return resp, nil
 	}