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.

(cherry picked from commit 83494605b2dd594ab33f9b3cfa5abc82cf0f9e92)

https://github.com/krb5/krb5/commit/d70597bc1cdd8bd22299b61ac17b9d684c626b8f
Author: Matt Rogers <mrogers@redhat.com>
Committer: Tom Yu <tlyu@mit.edu>
Commit: d70597bc1cdd8bd22299b61ac17b9d684c626b8f
Branch: krb5-1.13
 src/lib/kdb/kdb_default.c |    5 +++++
 1 files changed, 5 insertions(+), 0 deletions(-)