Restore the value of an environment variable properly if a withValue call yields.

See https://groups.google.com/forum/#!topic/meteor-core/DQ8Oc5s6phs.

To summarize, if `func()` yields, then `currentValues` may no longer point to `Fiber.current._meteor_dynamics`. So we need to make sure it does, otherwise `currentValues[this.slot] = saved` will be a no-op.