DCHECK for nullptrs in EventBaseLocal setter and factory getter

Summary: `getOrCreateFn()` is prone to undefined behavior as it returns `nullptr` reference if factory function returns a `nullptr` or empty `std::shared_ptr<T>`, or if `emplace(evb, nullptr)` is called. This diff adds `DCHECK`s to enforce non-nulls. Note that `getOrCreate()` is not affected as it uses `std::make_shared` under the hood that is guaranteed to have a new object constructed.

Differential Revision: D26246552

fbshipit-source-id: b7bc7164841b70fc0889c782438bf96a61857bbd

folly/io/async/EventBaseLocal.h

@@ -80,6 +80,7 @@ class EventBaseLocal : public detail::EventBaseLocalBase {
   T* get(EventBase& evb) { return static_cast<T*>(getVoid(evb)); }
 
   void emplace(EventBase& evb, T* ptr) {
+    DCHECK(ptr != nullptr);
     std::shared_ptr<T> smartPtr(ptr);
     setVoid(evb, std::move(smartPtr));
   }
@@ -110,6 +111,7 @@ class EventBaseLocal : public detail::EventBaseLocalBase {
       return *static_cast<T*>(ptr);
     }
     std::shared_ptr<T> smartPtr(fn());
+    DCHECK(smartPtr.get() != nullptr);
     auto& ref = *smartPtr;
     setVoid(evb, std::move(smartPtr));
     return ref;