Summary: During shutdown there's a race condition in IOThreadPoolExecutor::getEventBase where a thread is created in ensureActiveThreads, and then destroyed before pickThread() is called. This adds the same guard that exists in add().

Reviewed By: yfeldblum

Differential Revision: D17955227

fbshipit-source-id: 042cb5c42056a9da4578571fb10936bbc477c4ff