If there are no ready segments, leave the tx window alone

Before, we set it to size zero, preventing us from sending any further
data. See: #140

tcp/segment.ml

@@ -97,9 +97,9 @@ module Rx(Time:V1_LWT.TIME) = struct
       with Not_found -> false *)
 
   (* Determine the transmit window, from the last segment *)
-  let window q =
+  let window ~default q =
     try (S.max_elt q).window
-    with Not_found -> 0
+    with Not_found -> default
 
   let is_empty q = S.is_empty q.segs
 
@@ -140,7 +140,7 @@ module Rx(Time:V1_LWT.TIME) = struct
       let tx_ack =
         if seg.ack then begin
           StateTick.tick q.state (State.Recv_ack seg.ack_number);
-          let win = window ready in
+          let win = window ~default:(Window.ack_win q.wnd) ready in
           let data_in_flight = Window.tx_inflight q.wnd in
           let seq_has_changed = (Window.ack_seq q.wnd) <> seg.ack_number in
           let win_has_changed = (Window.ack_win q.wnd) <> win in