Prevent null dereference with keyboard master key

If krb5_db_fetch_mkey() prompts for a master key and needs to
determine the kvno, check that the master entry contains any key data
before dereferencing the first element.  Reported by Joshua Schaeffer.

https://github.com/krb5/krb5/commit/29c504504f0c56c861d968ba2498590bf34714cd
Author: Greg Hudson <ghudson@mit.edu>
Commit: 29c504504f0c56c861d968ba2498590bf34714cd
Branch: master
 src/lib/kdb/kdb5.c |    7 ++++---
 1 files changed, 4 insertions(+), 3 deletions(-)