gpg: Fix a NULL-deref due to empty ring trust packets.

* g10/parse-packet.c (parse_trust): Always allocate a packet.

Test data:

gpg2 --no-default-keyring --keyring FILE --export

With this unpacked data for FILE:

-----BEGIN PGP ARMORED FILE-----
Version: GnuPG v2
Comment: Use "gpg --dearmor" for unpacking

mI0EVNP2zQEEALvETPVDCJDBXkegF4esiV1fqlne40yJnCmJeDEJYocwFPXfFA86
sSGjInzgDbpbC9gQPwq91Qe9x3Vy81CkyVonPOejhINlzfpzqAAa3A6viJccZTwt
DJ8E/I9jg53sbYW8q+VgfLn1hlggH/XQRT0HkXMP5y9ClURYnTsNwJhXABEBAAG0
CXRlc3QgdGVzdIi5BBMBCgAjBQJU0/bNAhsDBwsJCAcDAgEGFQgCCQoLBBYCAwEC
HgECF4AACgkQlsmuCapsqYLvtQP/byY0tM0Lc3moftbHQZ2eHj9ykLjsCjeMDfPx
kZUUtUS3HQaqgZLZOeqPjM7XgGh5hJsd9pfhmRWJ0x+iGB47XQNpRTtdLBV/WMCS
l5z3uW7e9Md7QVUVuSlJnBgQHTS6EgP8JQadPkAiF+jgpJZXP+gFs2j3gobS0qUF
eyTtxs+wAAAD

puSt

-----END PGP ARMORED FILE-----

• Reported-by: Hanno Böck <hanno@hboeck.de>
• Signed-off-by: Werner Koch <wk@gnupg.org>