message: Fixed CreateStruct not testing for nesting.
  Tested: on Linux.
diff: @@ -1446,6 +1446,7 @@ class FlatBufferBuilder
  /// @brief Write a struct by itself, typically to be part of a union.
  template<typename T> Offset<const T *> CreateStruct(const T &structobj) {
  + NotNested();
  Align(AlignOf<T>());
  buf_.push_small(structobj);
  return Offset<const T *>(GetSize());
  