Clear the point S before freeing in ec_scalar_mul_ladder

The secret point R can be recovered from S using the equation R = S - P.
The X and Z coordinates should be sufficient for that.

Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/8504)

(cherry picked from commit 8a74bb5c)