http: fix IPv4 addresses not 4 bytes long

Fixes #230

frontend/http/parser.go

@@ -79,6 +79,11 @@ func ParseAnnounce(r *http.Request, realIPHeader string, allowIPSpoofing bool) (
 		return nil, bittorrent.ClientError("failed to parse peer IP address")
 	}
 
+	// Sanitize IPv4 addresses to 4 bytes.
+	if ip := request.Peer.IP.To4(); ip != nil {
+		request.Peer.IP = ip
+	}
+
 	return request, nil
 }