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.