Check princ length in krb5_sname_match()

krb5_sname_match() can read past the end of princ's component array in
some circumstances (typically when a keytab contains both "x" and
"x/y" principals).  Add a length check.  Reported by Spencer Jackson.

https://github.com/krb5/krb5/commit/fb9fcfa92fd37221c77e1a4c0b930383e6839e22
Author: Greg Hudson <ghudson@mit.edu>
Commit: fb9fcfa92fd37221c77e1a4c0b930383e6839e22
Branch: master
 src/lib/krb5/krb/sname_match.c |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)