Log service princ in KDC more reliably

Under some error conditions, the KDC would log "<unknown server>" for
the service principal because service principal information is not yet
available to the logging functions.  Set the appropriate variables
earlier.

do_as_req.c: After unparsing the client, immediately unparse the
server before searching for the client principal in the KDB.

do_tgs_req.c: Save a pointer to the client-requested service
principal, to make sure it gets logged if an error happens before
search_sprinc() successfully completes.

[tlyu@mit.edu: commit message; fix TGS to catch more error cases]

ticket: 7802
target_version: 1.12.1
tags: pullup
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.

ticket: 7802
Test bogus KDC-REQs

Send encodings that are invalid KDC-REQs, but pass krb5_is_as_req()
and krb5_is_tgs_req(), to make sure that the KDC recovers correctly
from failures in decode_krb5_as_req() and decode_krb5_tgs_req().  Also
send an encoding that isn't a valid KDC-REQ.

ticket: 7811 (new)
target_version: 1.12.1
tags: pullup