src: fix warning for potential snprintf truncation

gcc 8+ recognizes that space has not been left for the pid and that the
return value of snprintf() isn't checked. Leave a little space for the
pid to prevent `-Wformat-truncation`.

PR-URL: https://github.com/nodejs/node/pull/24810


Reviewed-By: Richard Lau <riclau@uk.ibm.com>