custom binary output port causes SEGV

[GoogleCodeExporter]

What steps will reproduce the problem?
1. Run the following script
#!r6rs
(import (rnrs))

(define (make-custom)
  (define (write! bv start count) -1)
  (define (close) #t)
  (make-custom-binary-output-port "custom" write! #f #f close))

(let ((out (make-custom)))
  (put-bytevector out #vu8(1 2 3 4 5)))


What is the expected output? What do you see instead?
Possibly &i/o error.

SEGV

What version of the product are you using? On what operating system?
% ypsilon --version
Ypsilon 0.9.6-update3 Copyright (c) 2008 Y.Fujita, LittleWing Company Limited.

Please provide any additional information below.

Original issue reported on code.google.com by ktakash...@gmail.com on 24 Oct 2014 at 7:02