Use the index that matches the key type (either SSL_PKEY_RSA_PSS_SIGN or SSL_PKEY_RSA).

Extract the RSA key using EVP_PKEY_get0.  Type is checked externally to be either EVP_PKEY_RSA_PSS or EVP_PKEY_RSA.

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4389)