gluten_lwt.ml / write_loop consumes 100% if remote side closes socket

function write_loop in lwt/gluten_lwt.ml fails to check the writev result. If writev writes to a socket, the socket has been closed by the remote side, and SIGPIPE has been disabled, writev will always return `Closed and write_loop will neither exit or yield, locking up the program.
I've fixed the problem by adding a check, but I'm not familiar enough with the code to do a PR.

```
match Runtime.next_write_operation t with
| `Write io_vectors ->
writev io_vectors >>= fun result ->
Runtime.report_write_result t result;
(* added check to exit loop on EPIPE *)
if result != `Closed then
write_loop_step ()
else
Lwt.return_unit
```