Fix unlikely pointer error in get_in_tkt.c

In add_padata(), reset the caller's pointer and ensure the list is
terminated as soon as realloc() succeeds; otherwise, the old pointer
could be left behind if a later allocation fails.

(cherry picked from commit 24452cd737951fa6e0f35e97c6a644a9db0aa82d)

https://github.com/krb5/krb5/commit/7c4a7adf7f224868c936cb56804ebd17d5d58756
Author: Greg Hudson <ghudson@mit.edu>
Committer: Tom Yu <tlyu@mit.edu>
Commit: 7c4a7adf7f224868c936cb56804ebd17d5d58756
Branch: krb5-1.13
 src/lib/krb5/krb/get_in_tkt.c |    6 +++---
 1 files changed, 3 insertions(+), 3 deletions(-)