avoid use of freed pointer

Without this patch, pk2 would be freed twice.

From 2a18a4b757e0896e738fefbbaa8ff8c23a9edf89 Mon Sep 17 00:00:00 2001

From: Jim Meyering <meyering@redhat.com>
Date: Tue, 20 Sep 2011 16:20:39 +0200
Subject: [PATCH] avoid use of freed pointer

If we free pk2 at the top of the for-loop, set it to NULL
so that we don't free it again just before returning.

• revoke.c (gen_desig_revoke): Don't use pk2 after freeing it.