Fix thread local random number generator static issue

Summary:
See https://gcc.gnu.org/onlinedocs/gcc-4.8.3/gcc/Thread-Local.html
Thanks to @subodh for finding the first instance of this problem in the
proxygen codebase.

Test Plan:
unit tests, tested proxygen unit tests on my laptop and I don't
get segfaults on program exit now.

Reviewed By: subodh@fb.com

Subscribers: trunkagent, doug, njormrod, subodh

FB internal diff: D1616149

Tasks: 5217022

folly/Random.cpp

@@ -98,8 +98,10 @@ void Random::secureRandom(void* data, size_t size) {
   bufferedRandomDevice->get(data, size);
 }
 
-folly::ThreadLocalPtr<ThreadLocalPRNG::LocalInstancePRNG>
-ThreadLocalPRNG::localInstance;
+ThreadLocalPRNG::ThreadLocalPRNG() {
+  static folly::ThreadLocal<ThreadLocalPRNG::LocalInstancePRNG> localInstance;
+  local_ = localInstance.get();
+}
 
 class ThreadLocalPRNG::LocalInstancePRNG {
  public:
@@ -108,12 +110,6 @@ class ThreadLocalPRNG::LocalInstancePRNG {
   Random::DefaultGenerator rng;
 };
 
-ThreadLocalPRNG::LocalInstancePRNG* ThreadLocalPRNG::initLocal() {
-  auto ret = new LocalInstancePRNG;
-  localInstance.reset(ret);
-  return ret;
-}
-
 uint32_t ThreadLocalPRNG::getImpl(LocalInstancePRNG* local) {
   return local->rng();
 }

folly/Random.h

@@ -64,18 +64,10 @@ class ThreadLocalPRNG {
   }
   friend class Random;
 
-  ThreadLocalPRNG() {
-    local_ = localInstance.get();
-    if (!local_) {
-      local_ = initLocal();
-    }
-  }
+  ThreadLocalPRNG();
 
  private:
   class LocalInstancePRNG;
-  static LocalInstancePRNG* initLocal();
-  static folly::ThreadLocalPtr<ThreadLocalPRNG::LocalInstancePRNG>
-    localInstance;
 
   static result_type getImpl(LocalInstancePRNG* local);
   LocalInstancePRNG* local_;