core: fix usage of uv_cwd

It was modified in libuv to be consistent with uv_exepath and not
include the trailing NULL byte in the returned size.

PR-URL: https://github.com/joyent/node/pull/8566


Reviewed-by: Fedor Indutny <fedor@indutny.com>
Reviewed-by: Trevor Norris <trev.norris@gmail.com>