Copy RSA-PSS saltlen in EVP_PKEY_CTX_dup.

pkey_rsa_copy was missing a field. Test this by repeating the operation
through an EVP_PKEY_CTX_dup copy in evp_test.

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/8759)