Fix a regression in error handling
    
    Commit fb6ffe0c50e166bf095736a051e4840bd5a5ad4f introduced a regression
    in acquire_cred_from() where a processing error would be masked and a
    GSS_COMPLETE status would be returned instead. This caused a credential
    structure to be returned when no credentials are actually available.
    
    Signed-off-by: Simo Sorce <simo@redhat.com>
    Reviewed-by: Greg Hudson <ghudson@mit.edu>