Double closed FD in async_pipe

I use `async_pipe` for communication between child and parent processes. When a child process finishes, then I destroy the `async_pipe`. The `async_pipe` destructor calls `::close()` on the both sides of the pipe, see:
```
~async_pipe()
{
if (_sink .native_handle() != -1)
::close(_sink.native_handle());
if (_source.native_handle() != -1)
::close(_source.native_handle());
}
```
However the destructor of `boost::asio::posix::stream_descriptor` (the type of `_sink` and `_source`) calls the other `::close()` on the file descriptor if it isn't `-1`. A closed FD in the `async_pipe` destructor isn't set to `-1` anywhere. The other `::close()` is called `/usr/local/include/boost/asio/detail/impl/descriptor_ops.ipp:49` in `boost::asio::detail::descriptor_ops::close()`. Call stack:
```
boost::asio::detail::descriptor_ops::close descriptor_ops.ipp:49
boost::asio::detail::reactive_descriptor_service::destroy reactive_descriptor_service.ipp:95
boost::asio::basic_io_object::~basic_io_object basic_io_object.hpp:247
boost::asio::posix::descriptor::~descriptor descriptor.hpp:629
boost::asio::posix::stream_descriptor::~stream_descriptor stream_descriptor.hpp:48
boost::process::detail::posix::async_pipe::~async_pipe async_pipe.hpp:71
```

Consequences: If a system hasn't reused the closed FD, then the other `::close()` returns `EBADF` which is ignored. Otherwise the other `::close()` closes a FD of someone else.