gpg: Fix a NULL-deref due to empty ring trust packets.
* g10/parse-packet.c (parse_trust): Always allocate a packet.
(back ported from commit 39978487863066e59bb657f5fe4e8baab510da7e)
[dkg: rebased to STABLE-BRANCH-1-4]
- Reported-by: Hanno Böck <hanno@hboeck.de>
- Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>