Fix uncommon PKINIT memory leak

Fix uncommon PKINIT memory leak

PKINIT per-request module data objects are normally created by
pkinit_server_verify_padata() and freed by
pkinit_server_return_padata().  In some unusual circumstances, the KDC
may not call the return_padata method after verification succeeds.
Add a free_modreq method and free the object there instead.

[ghudson@mit.edu: rewrote commit message]

ticket: 9065 (new)
tags: pullup
target_version: 1.20-next
target_version: 1.19-next