Reset offset when closing a channel

If offset keeps its value when the channel is closed, it is possible to
`seek_in` backward which will only decrease `channel->curr` and therefore
mark some buffer bytes as valid: further calls to `input_byte` would
succeed on the closed channel

Fix issue ocaml#11878

runtime/io.c

@@ -681,6 +681,9 @@ CAMLprim value caml_ml_close_channel(value vchannel)
      immediate caml_flush_partial or caml_refill, thus raising a Sys_error
      exception */
   channel->curr = channel->max = channel->end;
+  /* Prevent any seek backward that would mark the last bytes of the
+   * channel buffer as valid */
+  channel->offset = 0;
 
   /* If already closed, we are done */
   if (channel->fd != -1) {

testsuite/tests/lib-channels/close_in.ml

@@ -9,10 +9,6 @@ let () =
   seek_in ic nb_bytes;
   close_in ic;
   seek_in ic 0;
-  for _ = 1 to nb_bytes do
-    (* the bytes we get here were never initialised *)
-    ignore (input_byte ic)
-  done;
   assert (
     try
       ignore (input_byte ic);