test: fix out-of-bound reads from invalid sizeof usage

`sizeof(data)` does not return the correct result here, as it measures
the size of the `data` variable, not what it points to.

PR-URL: https://github.com/nodejs/node/pull/33115


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>