krb5 -- Double-free in KDC TGS processing

SO-AND-SO reports:

When issuing a ticket for a TGS renew or validate request, copy
only the server field from the outer part of the header ticket
to the new ticket. Copying the whole structure causes the
enc_part pointer to be aliased to the header ticket until
krb5_encrypt_tkt_part() is called, resulting in a double-free
if handle_authdata() fails…