src: allow getting Symbols on process.env

1aa595e5 introduced a `throw` for accessing `Symbol` properties of
`process.env`. However, this breaks `util.inspect(process)` and
things like `Object.prototype.toString.call(process.env)`, so this
patch changes the behaviour for the getter to just always return
`undefined`.

Ref: https://github.com/nodejs/node/pull/9446
Fixes: https://github.com/nodejs/node/issues/9641
PR-URL: https://github.com/nodejs/node/pull/9631


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>