Fix sync buffer crash

[songqing]

What do these changes do?

When test with mutable_fragment_tests, the process crashed when data is small. It seems the overloading of operator++(int) in code doesn't return the old value, but return the new value, which is not the normal implementation.
`
iterator operator++(int) noexcept {

  VID_T new_value = cur_.GetValue() + 1;
  if (new_value == head_end_) {
    new_value = tail_begin_;
  }
  return iterator(new_value, head_end_, tail_begin_);
}

`

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