RT 4393: Call EC_GROUP_order_bits in priv2opt.

The private key is a scalar and should be sized by the order, not the
degree. See RFC 5915.
Reviewed-by: NRich Salz <rsalz@openssl.org>
Reviewed-by: NEmilia Käsper <emilia@openssl.org>

crypto/ec/ec_key.c

@@ -607,7 +607,7 @@ size_t ec_key_simple_priv2oct(const EC_KEY *eckey,
 {
     size_t buf_len;
 
-    buf_len = (EC_GROUP_get_degree(eckey->group) + 7) / 8;
+    buf_len = (EC_GROUP_order_bits(eckey->group) + 7) / 8;
     if (eckey->priv_key == NULL)
         return 0;
     if (buf == NULL)