Remove inoperative null checks in KDC code

In prepare_error_tgs, kdc_free_rstate, and kdc_fast_handle_error,
remove unnecessary null checks.  The callers avoid passing null
states, and in each case we've already dereferenced the pointer
earlier.  Reported by Nickolai Zeldovich <nickolai@csail.mit.edu>.

https://github.com/krb5/krb5/commit/41b35299cbfa6a47e93f56344cd2e52dd4418ce6
Author: Greg Hudson <ghudson@mit.edu>
Commit: 41b35299cbfa6a47e93f56344cd2e52dd4418ce6
Branch: master
 src/kdc/do_tgs_req.c |    6 ++----
 src/kdc/fast_util.c  |    4 +---
 2 files changed, 3 insertions(+), 7 deletions(-)