fix handle leak in vp_pipe_open()

Also fix the possibility of double close problem. A handle returned by
_get_osfhandle() should not be passed to CloseHandle(), because it will
be closed automatically when closing the fd using _close().
Merge branch 'master' into topic/fix-handle-leak
fix CRT handle leak

vp_pipe_open() should close hstdin, hstdout and hstderr.
Merge pull request #130 from k-takata/topic/fix-handle-leak

fix handle leak in vp_pipe_open()