code: private X500Principal getCertIssuer(X509CRLEntryImpl entry,
          X500Principal prevCertIssuer) throws IOException {
  
          CertificateIssuerExtension ciExt =
              entry.getCertificateIssuerExtension();
          if (ciExt != null) {
              GeneralNames names = ciExt.get(CertificateIssuerExtension.ISSUER);
              X500Name issuerDN = (X500Name) names.get(0).getName();
              return issuerDN.asX500Principal();
          } else {
              return prevCertIssuer;
          }
      }
docs: Returns the X500 certificate issuer DN of a CRL entry.
  
  @param entry the entry to check
  @param prevCertIssuer the previous entry's certificate issuer
  @return the X500Principal in a CertificateIssuerExtension, or
  prevCertIssuer if it does not exist