PKCS#7: X.509 certificate issuer and subject are mandatory fields in the ASN.1

X.509 certificate issuer and subject fields are mandatory fields in the ASN.1
and so their existence needn't be tested for.  They are guaranteed to end up
with an empty string if the name material has nothing we can use (see
x509_fabricate_name()).

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Vivek Goyal <vgoyal@redhat.com>