message: Fixed DetachedBuffer self move assignment
diff: @@ -638,6 +638,9 @@ class DetachedBuffer {
  #if !defined(FLATBUFFERS_CPP98_STL)
  // clang-format on
  DetachedBuffer &operator=(DetachedBuffer &&other) {
  + if (this == &other)
  + return *this;
  +
  destroy();
  allocator_ = other.allocator_;
  