KEYS: Add safe guard against faulty PGP key

hulk inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I5H4FC


CVE: NA

--------------------------------

Under normal condition, when there is a user id packet, there will
always be a public key packet in the front, meaning ctx.fingerprint
will never be NULL. However, if a malicious or faulty PGP key is
provided with only user id packet but not public key packet, a read
out-of-bound will be triggered during the generation of key
description. To make things worse, a NULL pointer deference could be
triggered in pgp_key_generate_id().

This patch adds a safe guard which prevents parsing the key further if
no public key packet is provided.

Fixes: a98cb7a4 ("KEYS: Provide PGP key description autogeneration")
Signed-off-by: GUO Zihua <guozihua@huawei.com>
Reviewed-by: Roberto Sassu <roberto.sassu@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>