Fix panic in connection.iterPendingPieces when the info isn't available
Fix panic in connection.iterPendingPieces when the info isn't available

Triggered by connection.iterUnbiasedPieceRequestOrder calling Torrent.numPieces. We shouldn't be iterating pieces when there's no info anyway, so don't yield any. Mentioned in https://github.com/anacrolix/torrent/issues/253#issuecomment-400350386.

anacrolix/torrent,anacrolix/torrent