pkcs7: fix check for self-signed certificate

pkcs7_validate_trust_one() used 'x509->next == x509' to identify a
self-signed certificate.  That's wrong; ->next is simply the link in the
linked list of certificates in the PKCS#7 message.  It should be
checking ->signer instead.  Fix it.

Fortunately this didn't actually matter because when we re-visited
'x509' on the next iteration via 'x509->signer', it was already seen and
not verified, so we returned -ENOKEY anyway.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: James Morris <james.l.morris@oracle.com>