Torrent.Metainfo: Don't return incomplete metadata bytes

Fixes #208

torrent.go

@@ -548,7 +548,13 @@ func (t *Torrent) newMetaInfo() metainfo.MetaInfo {
 		Comment:      "dynamic metainfo from client",
 		CreatedBy:    "go.torrent",
 		AnnounceList: t.metainfo.UpvertedAnnounceList(),
-		InfoBytes:    t.metadataBytes,
+		InfoBytes: func() []byte {
+			if t.haveInfo() {
+				return t.metadataBytes
+			} else {
+				return nil
+			}
+		}(),
 	}
 }