gpg: Fix off-by-one read in the attribute subpacket parser.

* g10/parse-packet.c (parse_attribute_subpkts): Check that the
attribute packet is large enough for the subpacket type.

(backported from commit 0988764397f99db4efef1eabcdb8072d6159af76)

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