Downloader: handle nil-possibilities

[AskAlexSharov]

in next code pattern many things can be nil:

if info, err := d.torrentInfo(ts.DisplayName); err == nil {
	if info.Completed != nil {
		fi, serr := os.Stat(filepath.Join(d.SnapDir(), info.Name))
		if serr != nil || fi.Size() != *info.Length || !fi.ModTime().Equal(*info.Completed) {
			if err := d.db.Update(d.ctx, torrentInfoReset(info.Name, info.Hash, *info.Length)); err != nil {
				if serr != nil {
					log.Error("[snapshots] Failed to reset db entry after stat error", "file", info.Name, "err", err, "stat-err", serr)
				} else {
					log.Error("[snapshots] Failed to reset db entry after stat mismatch", "file", info.Name, "err", err)
				}
			}
		}
	}
}

possible nils: info, fi.