[crypto] Fix unused-but-set variable warning

Reported-by: Brandon Penglase <bpenglase-ipxe@spaceservices.net>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/validator.c

  */
 static void validator_step ( struct validator *validator ) {
 	struct x509_link *link;
-	struct x509_link *previous;
 	struct x509_certificate *cert;
 	struct x509_certificate *issuer = NULL;
 	struct x509_certificate *last;
 	list_for_each_entry ( link, &validator->chain->links, list ) {
 		cert = issuer;
 		issuer = link->cert;
-		previous = link;
 		if ( ! cert )
 			continue;
 		if ( ! issuer->valid )