Fix fd leak noticed by @hannesm
Fix fd leak noticed by @hannesm

Some closing code is copy pasted from Conduit_lwt_unix but since no code
is being shared at all currently, it's ok for now.

rgrinberg/ocaml-conduit,djs55/ocaml-conduit,djs55/ocaml-conduit,avsm/ocaml-conduit,samoht/ocaml-conduit,edwintorok/ocaml-conduit,mirage/ocaml-conduit,samoht/ocaml-conduit,rgrinberg/ocaml-conduit,edwintorok/ocaml-conduit