[libc++] std::out_of_range thrown when calling str() && on default-constructed ostringstream

```cpp
// ./bin/clang++ -std=c++20 /tmp/test.cc -g
#include <iostream>
#include <sstream>

int main() {
  std::ostringstream oss;
  std::cerr << std::move(oss).str();
  return 0;
}
```

fails with `libc++abi: terminating due to uncaught exception of type std::out_of_range: basic_string`
