Cleanup: need int64 cast to prevent overflow on the 32-bit architecture

This PR is a trivial fix.

Per #210 , there will be cross-compiling for both `amd64` and `386` archs. A test failed because an `int64` value needs an explicit cast on the `386` arch.

Signed-off-by: Chanwit Kaewkasi <chanwit@gmail.com>