[Seckrb5] Avoid null pointer dereference (xrootd#2385)

Author: smithdh

src/XrdSeckrb5/XrdSecProtocolkrb5.cc

@@ -510,7 +510,9 @@ int XrdSecProtocolkrb5::Authenticate(XrdSecCredentials *cred,
           {char* cpName;
            int ec;
            isCP = true;
-           if ((ec = krb5_unparse_name(krb_context,
+           if (!Ticket || !Ticket->enc_part2)
+               cPrincipal = "[principal not available]";
+           else if ((ec = krb5_unparse_name(krb_context,
                           (krb5_const_principal)Ticket->enc_part2->client,
                           (char **)&cpName)))
               {char mBuff[1024];