util: use assert for unreachable code

Great effort is taken to keep `util.inspect()` from throwing while
inspecting a linked list or other items that require recursion. It is
likely still possible at this time to cause a throw with prototype
pollution, but that will change soon when the code is modified to use
primordials.

This commit instructs our coverage to not be concerned about not being
able to reach the throwing-an-error code and changes the throw there to
an assert so users will be instructed to open a bug with Node.js should
they manage to cause the code to throw.

PR-URL: https://github.com/nodejs/node/pull/37249


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>