Use BN_clear_free in DH_set0_key

Reviewed-by: NPaul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/9796)

(cherry picked from commit fa01370f7dc8f0a379483bbe74de11225857e5fe)

crypto/dh/dh_lib.c

@@ -234,11 +234,11 @@ void DH_get0_key(const DH *dh, const BIGNUM **pub_key, const BIGNUM **priv_key)
 int DH_set0_key(DH *dh, BIGNUM *pub_key, BIGNUM *priv_key)
 {
     if (pub_key != NULL) {
-        BN_free(dh->pub_key);
+        BN_clear_free(dh->pub_key);
         dh->pub_key = pub_key;
     }
     if (priv_key != NULL) {
-        BN_free(dh->priv_key);
+        BN_clear_free(dh->priv_key);
         dh->priv_key = priv_key;
     }