Change to remote address introduced bug that crashes the server if client disconnects

[rix0rrr]

The following change to http_server_asio.cpp introduced an issue:

2dcf4fd#diff-d8600089329e519f56170a2d16d7102c

When the client connects and immediately disconnects, remote_endpoint().address() will throw an exception (ENOTCONN):

terminate called after throwing an instance of 'boost::exception_detail::clone_impl<boost::exception_detail::error_info_injector<boost::system::system_error> >'
  what():  remote_endpoint: Transport endpoint is not connected

This crashes the server.