crypto: fix DiffieHellman `generator` validation

PR-URL: https://github.com/nodejs/node/pull/38311
Fixes: https://github.com/nodejs/node/issues/38302


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