Add missing vendor into vendor.conf.

Signed-off-by: Lantao Liu <lantaol@google.com>

vendor/github.com/json-iterator/go/feature_pool.go

@@ -28,6 +28,7 @@ func (cfg *frozenConfig) BorrowStream(writer io.Writer) *Stream {
 
 func (cfg *frozenConfig) ReturnStream(stream *Stream) {
 	stream.Error = nil
+	stream.Attachment = nil
 	select {
 	case cfg.streamPool <- stream:
 		return
@@ -48,6 +49,7 @@ func (cfg *frozenConfig) BorrowIterator(data []byte) *Iterator {
 
 func (cfg *frozenConfig) ReturnIterator(iter *Iterator) {
 	iter.Error = nil
+	iter.Attachment = nil
 	select {
 	case cfg.iteratorPool <- iter:
 		return