Fix a memory leak in krb5_get_init_creds_keytab

lookup_etypes_for_keytab was not freeing keytab entries during its
iteration over the keytab.  Extract the information we need from each
entry and free the entry before continuing.  Reported by
nalin@redhat.com.

ticket: 7586
target_version: 1.11.1
tags: pullup