[Bug libstdc++/123100] std::ostringstream:str() (r-value version) returns garbage

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=123100

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---

basic_stringbuf is implemented like:
```
      _GLIBCXX_NODISCARD
      __string_type
      str() &&
      {
        if (char_type* __hi = _M_high_mark())
          {
            // Set length to end of character sequence and add null terminator.
            _M_string._M_set_length(_M_high_mark() - this->pbase());
          }
        auto __str = std::move(_M_string);
        _M_string.clear();
        _M_sync(_M_string.data(), 0, 0);
        return __str;
      }

```

Maybe the type of __str is wrong ...