buffer: remove obsolete NaN check

These two NaN entries are not necessary and we can safely remove them.

PR-URL: https://github.com/nodejs/node/pull/18744


Reviewed-By: Weijia Wang <starkwang@126.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>