src: fix setting Converter sub char length

Signed-off-by: James M Snell <jasnell@gmail.com>
Fixes: https://github.com/nodejs/node/issues/38330

PR-URL: https://github.com/nodejs/node/pull/38331


Reviewed-By: Juan José Arboleda <soyjuanarbol@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>