[InterNetNews/inn] d1f571: Fix (unlikely) possible double free in Cancel-Lock

[Julien ÉLIE]

  Branch: refs/heads/main
  Home:   https://github.com/InterNetNews/inn
  Commit: d1f571f6bbd02a3cd5ee454bb27328c568bbd053
      https://github.com/InterNetNews/inn/commit/d1f571f6bbd02a3cd5ee454bb27328c568bbd053
  Author: Julien ÉLIE <Julien-Elie at users.noreply.github.com>
  Date:   2022-01-11 (Tue, 11 Jan 2022)

  Changed paths:
    M lib/canlock.c
    M nnrpd/post.c

  Log Message:
  -----------
  Fix (unlikely) possible double free in Cancel-Lock

After freeing the canlock buffer, set it to NULL.  Otherwise, it might be
double freed in an unlikely case that a user posts 3 articles in the same
NNTP session, and libcanlock failed to generate lock hashes for the 2nd
article (but managed for the 1st).  When trying to send the 3rd article,
the canlock buffer will be freed, though unallocated.