Description of problem:

The configuration with SPNEGO works fine, however from time to time the authentication fails with the following error:


ERROR (HTTP-341)  [org.jboss.security.auth.spi.AbstractServerLoginModule] Unable to authenticate: java.lang.NullPointerException
        at org.jboss.security.negotiation.spnego.SPNEGOLoginModule$AcceptSecContext.run(SPNEGOLoginModule.java:420)
        at java.security.AccessController.doPrivileged(Native Method)
        at javax.security.auth.Subject.doAs(Subject.java:356)


Version-Release number of selected component (if applicable):

JBoss Security Negotiation 2.3.3.Final

How reproducible:
This happens very rarely (20 times in a day on a system where about 50 users are working) and it is extremely hard to reproduce. 

Steps to Reproduce:
1.
2.
3.

Actual results:


Expected results:


Additional info:

At line 420 in [1], the GSSToken is null
~~~~
           if (respToken != null)
            {
               NegotiationMessage response;
               if (requestMessage instanceof KerberosMessage)
               {
                  response = new KerberosMessage(Constants.KERBEROS_V5, respToken);
               }
               else
               {
                  NegTokenTarg negTokenTarg = new NegTokenTarg();
                  negTokenTarg.setResponseToken(respToken);

                  response = negTokenTarg;
               }
~~~~

It looks like a GSSToken can be or is null, check the line#344 as follows:-
~~~~~~~~~
public Object run()
      {        
         try
         {
            // The message type will have already been checked before this point so we know it is
            // a SPNEGO message.
            NegotiationMessage requestMessage = negotiationContext.getRequestMessage();

            // TODO - Ensure no way to fall through with gssToken still null.
            byte[] gssToken = null;
            if (requestMessage instanceof NegTokenInit)
            {
            ...
~~~~~~~~~

[1] : https://github.com/wildfly-security/jboss-negotiation/blob/2.3.3.Final/jboss-negotiation-spnego/src/main/java/org/jboss/security/negotiation/spnego/SPNEGOLoginModule.java