Fix crashes in readRandomDevice()

Fix crashes in readRandomDevice()

Summary: The comment in readRandomDevice() lies - the PCHECK fails with "Bad file descriptor" in some tests. This is most likely due to a race where a background thread is calling readRandomDevice() while the main thread shuts down. Another possibility is bad destruction order, where rnadomDevice is destroyed before some other static object whose destructor calls readRandomDevice(). Either way, this fixes it without having to chace down every instance.

Test Plan: run unit tests

Reviewed By: meyering@fb.com

Subscribers: antonl, trunkagent, meyering, simpkins, njormrod, folly-diffs@, tao-eng@

FB internal diff: D1660903

Signature: t1:1660903:1415215895:7f1f96ab7416b45c4dc18e78173fb59eb27bd03b