code: public IssuerAlternativeNameExtension getIssuerAltNameExtension()
          throws IOException {
          Object obj = getExtension(PKIXExtensions.IssuerAlternativeName_Id);
          return (IssuerAlternativeNameExtension)obj;
      }
docs: return the IssuerAlternativeNameExtension, if any.
  
  @returns IssuerAlternativeNameExtension or null (if no such extension)
  @throws IOException on error