[core] Make sure ThreadLocal will not own the pointer it is managing

ThreadLocal should not own the pointer it is managing because
the use case in Mapbox GL is to keep a pointer to a stack allocated
object, like:

```
MyObject foo;
threadLocal.set(&foo);
```

To keep consistency, it is required that we clear the managed
object before ThreadLocal gets destroyed by setting it to `nullptr`.

platform/default/thread_local.cpp

@@ -29,7 +29,11 @@ ThreadLocal<T>::ThreadLocal() : impl(std::make_unique<Impl>()) {
 
 template <class T>
 ThreadLocal<T>::~ThreadLocal() {
-    delete get();
+    // ThreadLocal will not take ownership
+    // of the pointer it is managing. The pointer
+    // needs to be explicitly cleared before we
+    // destroy this object.
+    assert(!get());
 
     if (pthread_key_delete(impl->key)) {
         Log::Error(Event::General, "Failed to delete local storage key.");