X.509: Fix error code in x509_cert_parse()

We forgot to set the error code on this path so it could result in
returning NULL which leads to a NULL dereference.

Fixes: db6c43bd ("crypto: KEYS: convert public key and digsig asym to the akcipher api")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: James Morris <james.l.morris@oracle.com>