X.509: Fix crash caused by NULL pointer

X.509: Fix crash caused by NULL pointer

Origin: https://lore.kernel.org/linux-crypto/20210107092855.76093-1-tianjia.zhang@linux.alibaba.com/
Bug-Debian: https://bugs.debian.org/979496

On the following call path, `sig->pkey_algo` is not assigned
in asymmetric_key_verify_signature(), which causes runtime
crash in public_key_verify_signature().

  keyctl_pkey_verify
    asymmetric_key_verify_signature
      verify_signature
        public_key_verify_signature

This patch simply check this situation and fixes the crash
caused by NULL pointer.

Fixes: 215525639631 ("X.509: support OSCCA SM2-with-SM3 certificate verification")
Cc: stable@vger.kernel.org # v5.10+
Reported-by: Tobias Markus <tobias@markus-regensburg.de>
Signed-off-by: Tianjia Zhang <tianjia.zhang@linux.alibaba.com>
Gbp-Pq: Topic bugfix/all
Gbp-Pq: Name X.509-Fix-crash-caused-by-NULL-pointer.patch