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)

crypto/ec/ec_mult.c

@@ -378,7 +378,7 @@ int ec_scalar_mul_ladder(const EC_GROUP *group, EC_POINT *r,
 
  err:
     EC_POINT_free(p);
-    EC_POINT_free(s);
+    EC_POINT_clear_free(s);
     BN_CTX_end(ctx);
 
     return ret;