Fix possible null deref in previous

My rework of the do_tgs_req.c patch introduced a null deref if
decode_krb5_tgs_req() failed.

(cherry picked from commit 30589b2a1636de9f9b68591f0e546cb0fa21989f)

https://github.com/krb5/krb5/commit/8acb855e63260fa4d114563107fcc776b3446336
Author: Tom Yu <tlyu@mit.edu>
Commit: 8acb855e63260fa4d114563107fcc776b3446336
Branch: krb5-1.11
 src/kdc/do_tgs_req.c |    7 ++++---
 1 files changed, 4 insertions(+), 3 deletions(-)