Fix KDC memory leak on failed S4U2Proxy requests

Make sure to release stkt_server in process_tgs_req() if we fail
before its disposition is determined.  Reported by Will Fiveash.

(back ported from commit 194641a8ad7aecc6bc1d4848742c14569f14b900)

https://github.com/krb5/krb5/commit/c16a6bb0de6b09319d8bb80b04c5bf87b31798f2
Author: Greg Hudson <ghudson@mit.edu>
Committer: Tom Yu <tlyu@mit.edu>
Commit: c16a6bb0de6b09319d8bb80b04c5bf87b31798f2
Branch: krb5-1.13
 src/kdc/do_tgs_req.c |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)