When the hash algorithm for the signature is not available the digest size
is 0 and the signature in the certificate is marked as unsupported.

When validating a self-signed certificate, this needs to be checked,
because otherwise trying to validate the signature will fail with an
warning:

Loading compiled-in X.509 certificates
WARNING: CPU: 0 PID: 1 at crypto/rsa-pkcs1pad.c:537 \
pkcs1pad_verify+0x46/0x12c
...
Problem loading in-kernel X.509 certificate (-22)
Signed-off-by: Thore Sommer <public@thson.de>
Cc: stable@vger.kernel.org # v4.7+
Fixes: 6c2dc5ae ("X.509: Extract signature digest and make self-signed cert checks earlier")
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>