param_bld: add a padded BN call.

param_bld: add a padded BN call.

To aviod leaking size information when passing private value using the
OSSL_PARAM builder, a padded BN call is required.

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/10840)