Fix generating random bytes under Windows

Summary: I'm hopefully the only user of this.

Test Plan:

$ arc unit

See no more:

Uninitialized string offset: 19 at [libphutil/src/filesystem/Filesystem.php:394]

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5029