Fix KDC to drop repeated in-progress requests

When a KDC receives a repeated request while the original request is
still in progress, it is supposed to be to drop the request.  Commit
f07760088b72a11c54dd72efbc5739f231a4d4b0 introduced a bug in this
logic, causing the KDC to instead send an empty reply.  In
kdc_check_lookaside(), return a NULL reply_packet for empty entries,
restoring the expected behavior.

[ghudson@mit.edu: edited commit message, added a comment]

https://github.com/krb5/krb5/commit/847fc7b3caa823c219c97cc307ccb8d7d519a20f
Author: Sarah Day <sarahday@mit.edu>
Committer: Greg Hudson <ghudson@mit.edu>
Commit: 847fc7b3caa823c219c97cc307ccb8d7d519a20f
Branch: master
 src/kdc/replay.c |    5 +++++
 1 files changed, 5 insertions(+), 0 deletions(-)