Fix infinite recursion in `non_null`

Summary: We need a more systematic way to deal with this, but let's fix
the worst one now.

Reviewed By: @elgenie

Differential Revision: D1743486
Update vfsstream framework test.

Summary: Update vfsstream framework test to include a fix for
vfsStreamWrapperWithoutRootTestCase::canNotOpen.
Closes https://github.com/facebook/hhvm/pull/4438

Reviewed By: @JoelMarcey

Differential Revision: D1737384

Signature: t1:1737384:1418767465:8f3c57e189d7bafae159dcfdd7012e712d28d1cc