remove state change after Flush

A call do Flush does not necessarily mean that the state should
change.  Another write could come in, if the state is closed this
can't occur.  Another Write can come in, it just needs to be a new
block.

writer.go

@@ -153,7 +153,7 @@ func (w *Writer) Flush() (err error) {
 	default:
 		return nil
 	}
-	defer w.state.nextd(&err)
+
 	if w.idx > 0 {
 		// Flush pending data, disable w.data freeing as it is done later on.
 		if err = w.write(w.data[:w.idx], false); err != nil {