Fix krb5_def_fetch_mkey_list() segfault

Return KRB5_KDB_NOMASTERKEY if K/M contains no key data, instead of
blindly dereferencing the first key data element.

https://github.com/krb5/krb5/commit/83494605b2dd594ab33f9b3cfa5abc82cf0f9e92
Author: Matt Rogers <mrogers@redhat.com>
Committer: Greg Hudson <ghudson@mit.edu>
Commit: 83494605b2dd594ab33f9b3cfa5abc82cf0f9e92
Branch: master
 src/lib/kdb/kdb_default.c |    5 +++++
 1 files changed, 5 insertions(+), 0 deletions(-)