Make ThreadLocalPtr::Accessor::Iterator default construct as singular

Reviewed By: yfeldblum, mshneer

Differential Revision: D29578762

fbshipit-source-id: 2aa9af8f68825997db8b71fc97dda5477fa57413

folly/ThreadLocal.h

@@ -253,8 +253,8 @@ class ThreadLocalPtr {
     // The iterators obtained from Accessor are bidirectional iterators.
     class Iterator {
       friend class Accessor;
-      const Accessor* accessor_;
-      threadlocal_detail::ThreadEntryNode* e_;
+      const Accessor* accessor_{nullptr};
+      threadlocal_detail::ThreadEntryNode* e_{nullptr};
 
       void increment() {
         e_ = e_->getNext();
@@ -311,6 +311,8 @@ class ThreadLocalPtr {
       using pointer = T const*;
       using iterator_category = std::bidirectional_iterator_tag;
 
+      Iterator() = default;
+
       Iterator& operator++() {
         increment();
         return *this;