Fix memory leak on error in KDC decrypt_2ndtkt()

Make sure to release the server principal entry in the cleanup handler
if it is not assigned to the output parameter.  Reported by Will
Fiveash.

https://github.com/krb5/krb5/commit/a1faaa4d6a404e3103f45e639b8890c3b141dfe1
Author: Greg Hudson <ghudson@mit.edu>
Commit: a1faaa4d6a404e3103f45e639b8890c3b141dfe1
Branch: master
 src/kdc/do_tgs_req.c |    4 +++-
 1 files changed, 3 insertions(+), 1 deletions(-)