src: turn key length exception into CHECK

This exception can logically never happen because of the key stretching
that takes place first.  Failure must therefore be a bug in Node.js and
not in the executing script.

PR-URL: https://github.com/nodejs/node/pull/15183


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>