Fix one more race in Promise::setTry

Summary:
Baton::post has to be the last step of Promise::setTry. If Promise is not owned by
the posting thread, it may be destroyed right after Baton::post is called.

Reviewed By: pavlo-fb

Differential Revision: D3312506

fbshipit-source-id: 6d5c3f0925c3bb9cd3f981e7550f888d5ed76189

folly/experimental/fibers/Promise-inl.h

@@ -63,9 +63,9 @@ void Promise<T>::setTry(folly::Try<T>&& t) {
   *value_ = std::move(t);
   value_ = nullptr;
 
+  // Baton::post has to be the last step here, since if Promise is not owned by
+  // the posting thread, it may be destroyed right after Baton::post is called.
   baton_->post();
-
-  baton_ = nullptr;
 }
 
 template <class T>