Strange code in subversion/libsvn_client/blame.c:file_rev_handler()

I've recently encountered strange code in
subversion/libsvn_client/blame.c:file_rev_handler():
[[[
....
  /* Prepare the text delta window handler. */
  if (frb->last_filename)
    SVN_ERR(svn_stream_open_readonly(&delta_baton->source_stream,
frb->last_filename,
                                     frb->currpool, pool));
  else
    /* Means empty stream below. */
    delta_baton->source_stream = NULL;
  last_stream = svn_stream_disown(delta_baton->source_stream, pool);
....
  /* Handle all delta - even if it is empty.
     We must do the latter to "merge" blame info from other branches. */
  if (content_delta_handler)
    {
      /* Proper delta - get window handler for applying delta.
         svn_ra_get_file_revs2 will drive the delta editor. */
      svn_txdelta_apply(last_stream, cur_stream, NULL, NULL,
                        frb->currpool,
                        &delta_baton->wrapped_handler,
                        &delta_baton->wrapped_baton);
      *content_delta_handler = window_handler;
      *content_delta_baton = delta_baton;
    }
  else
    {
      /* Apply an empty delta, i.e. simply copy the old contents.
         We can't simply use the existing file due to the pool rotation logic.
         Trigger the blame update magic. */
      SVN_ERR(svn_stream_copy3(last_stream, cur_stream, NULL, NULL, pool));
      SVN_ERR(update_blame(delta_baton));
    }
]]]]

As far I understand svn_stream_disown() could be called for NULL
stream, later this stream (LAST_STREAM) used as parameter for
svn_txdelta_apply() or svn_stream_copy3().

The question why it doesn't crash? And how this code supposed to work?

Bert, do you have any ideas? I see you change this code multiple times recently.

-- 
Ivan Zhakov