checking if we build torrent file

[enriavil1]

Sometimes we don't need to build torrent files , which return nil inside of BuildTorrentFileIfNeed. This means that sometimes we try to convert a Proto file to an info hash, but we don't have one throwing a nil pointer.

[AskAlexSharov]

We can’t just ignore it, because we need seed this files for other peers
It happening because I moved AddSegment after Proto2InfoHas. Let’s try call Proto2InfoHash only if TorrentHash is not nil.

[AskAlexSharov]

Looks great, but let’s avoid empty hashes.