GramJob doesn't behave properly when destroying delegated credentials of
subjobs of a multijob.

GramJob.destroyDelegatedCredentials():

...
EndpointReferenceType subJobCredentialEndpoint
              = subJobDescriptions[index].getJobCredentialEndpoint();
if (jobCredentialEndpoint != null)
{
    destroyDelegatedCredential(subJobCredentialEndpoint);
}
...

The if statement must check for subJobCredentialEndpoint here instead of
jobCredentialEndpoint.