src: fix vector subscript out of range

PR-URL: https://github.com/nodejs/node/pull/18460
Fixes: https://github.com/nodejs/node/issues/18459


Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Nikolai Vavilov <vvnicholas@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Khaidi Chu <i@2333.moe>