[crypto] Fail fast if cross-certificate source is empty

In fully self-contained deployments it may be desirable to build iPXE
with an empty CROSSCERT source to avoid talking to external services.

Add an explicit check for this case and make validator_start_download
fail immediately if the base URI is empty.

Signed-off-by: Ladi Prosek <lprosek@redhat.com>
Modified-by: Michael Brown <mcb30@ipxe.org>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/validator.c

 	/* Determine cross-signed certificate source */
 	fetch_string_setting_copy ( NULL, &crosscert_setting, &crosscert_copy );
 	crosscert = ( crosscert_copy ? crosscert_copy : crosscert_default );
+	if ( ! crosscert[0] ) {
+		rc = -EINVAL;
+		goto err_check_uri_string;
+	}
 
 	/* Allocate URI string */
 	uri_string_len = ( strlen ( crosscert ) + 22 /* "/%08x.der?subject=" */
  err_open_uri_string:
 	free ( uri_string );
  err_alloc_uri_string:
+ err_check_uri_string:
 	free ( crosscert_copy );
 	return rc;
 }