util/sss_krb5: find_principal_in_keytab() was amended
    
     - do not treat failure of krb5_kt_end_seq_get() as an fatal error
     - avoid calling sss_krb5_free_keytab_entry_contents(null_struct)
       as libkrb5 api docs do not specify explicitly behaviour in this case
    
    Reviewed-by: Sumit Bose <sbose@redhat.com>