certificate w/o issuer can't be parsed

[victorjulien]

Rule writers have reported that Suricata can't inspect some fields in a cert if the issuer is missing from it. https://redmine.openinfosecfoundation.org/issues/5439

Suricata's call to parse_x509_der errors with: (suricata::x509::rs_x509_decode) -- Error(Der(NomError(Many1)))

Suricata currently uses 0.11 in our git master.