src: use sizeof(var) instead of sizeof(type)

Change `memset(&s, 0, sizeof(type))` to `memset(&s, 0, sizeof(s))`.
The former is dangerous when the type of `s` changes.

PR-URL: https://github.com/nodejs/node/pull/27038


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>