locking: Set the lock_ctx->request to NULL when request is freed

locking: Set the lock_ctx->request to NULL when request is freed

The code was added to ctdb_lock_context_destructor() to ensure that
the if a lock_ctx gets freed first, the lock_request does not have a
dangling pointer.  However, the reverse is also true.  When a lock_request
is freed, then lock_ctx should not have a dangling pointer.

In commit 374cbc7b0ff68e04ee4e395935509c7df817b3c0, the code for second
condition was dropped causing a regression.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=11293

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Amitay Isaacs <amitay@gmail.com>
(Imported from commit 752ec31bcbbfe9f5b3b1c5dde4179d69f41cb53c)