assert: fix infinite loop

In rare cirumstances it is possible to get a identical error diff.
In such a case the advances diffing runs into a infinite loop.
This fixes it by properly checking for extra entries.

PR-URL: https://github.com/nodejs/node/pull/18611


Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>