src: simplify MaybeStackBuffer::capacity()

PR-URL: https://github.com/nodejs/node/pull/33602


Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Zeyu Yang <himself65@outlook.com>
Reviewed-By: Juan José Arboleda <soyjuanarbol@gmail.com>
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>