Fix KDC to drop repeated in-progress requests

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]

(cherry picked from commit 847fc7b3caa823c219c97cc307ccb8d7d519a20f)

ticket: 8477
version_fixed: 1.13.7