test/params_api_test.c: fix size_t assumptions

size_t isn't always as large as a int64_t, so the compiler complains
about possible data loss.  In this case, we are in control, so a
simple cast will do.

Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
(Merged from https://github.com/openssl/openssl/pull/8497)