Wrong conditions in retain_traits::decrement

[KalleOlaviNiemitalo]

In template <class T> struct retain_traits, the decrement functions seem to delete the object in the wrong situations:

  static void decrement (atomic_reference_count<T>* ptr) noexcept {
    ptr->count.fetch_sub(1, std::memory_order_acq_rel);
    if (not use_count(ptr)) { delete static_cast<T*>(ptr); }
}

Suppose you have two retain_ptr smart pointers pointing to the same object and its reference count is 2. Then, both retain_ptr smart pointers are destructed in different threads. In each thread, fetch_sub decrements the reference count, so it goes from 2 to 1 and then to 0. Then in each thread, use_count returns 0 and both threads delete the same object. decrement should instead check what value fetch_sub returned, so that only the thread that changed the reference count to 0 deletes the object.

  static void decrement (reference_count<T>* ptr) noexcept {
    if (ptr->count -= 1) { delete static_cast<T*>(ptr); }
}

This seems to delete the object when the reference count decreases to any nonzero value. It should delete when the reference count decreases to zero. Commit 898a4fd in PR #6 fixes that.