pgp: Check result of crypto_alloc_shash properly

hulk inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I5Z7MD


CVE: NA

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Return of crypto_alloc_shash would be either a valid pointer or an error
pointer. We should check the result with IS_ERR.

Signed-off-by: GUO Zihua <guozihua@huawei.com>
Reviewed-by: Xiu Jianfeng <xiujianfeng@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>