creds pointer needs to be set to NULL after free

There is an bug fix in our bug pool related to creds pointer not set to NULL after freed. The bug fix is still applicable to MIT kerberos.

172cleanup:
173    if (retval)
174        krb5_free_tgt_creds(context, *pppcreds);
175
176cleanup_cred:
177    krb5_free_cred(context, pcred);
178    krb5_free_cred_enc_part(context, &encpart);
179
180    return retval;
181}

*pppcreds should be set to NULL after krb5_free_tgt_creds()