Guard access to Fiber's Thread. Fixes #3772.
Guard access to Fiber's Thread. Fixes #3772.

The Fiber's Thread can disappear any time the Fiber has been suspended.

kachick/rubinius,kachick/rubinius,kachick/rubinius,kachick/rubinius,kachick/rubinius,kachick/rubinius,kachick/rubinius,kachick/rubinius