util: fix out of bounds indices in util.inspect

This fixes a issue brought up in #15288.

PR-URL: https://github.com/nodejs/node/pull/14881
Refs: https://github.com/nodejs/node/issues/15288


Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>