Fix race between PrimaryPtr::lock and PrimaryPtr::join

Reviewed By: andriigrynenko

Differential Revision: D33532498

fbshipit-source-id: bd180566563576d003afced43684a8e0cb8e3cee

folly/experimental/PrimaryPtr.h

@@ -164,9 +164,6 @@ class PrimaryPtr {
   // was called or the cleanup() task was started (even if the call to join()
   // hasn't returned yet and the cleanup() task has not completed yet).
   std::shared_ptr<T> lock() const {
-    if (!*this) {
-      return nullptr;
-    }
     if (auto outerPtr = outerPtrWeak_.lock()) {
       return *outerPtr;
     }