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.

(cherry picked from commit fb9fcfa92fd37221c77e1a4c0b930383e6839e22)

https://github.com/krb5/krb5/commit/831140bfe5580006095535447afb72502216004d
Author: Greg Hudson <ghudson@mit.edu>
Committer: Tom Yu <tlyu@mit.edu>
Commit: 831140bfe5580006095535447afb72502216004d
Branch: krb5-1.14
 src/lib/krb5/krb/sname_match.c |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)