win32.c: fix pipe name formatting

* win32/win32.c (rb_w32_pipe): fix pipe name formatting.  as "%x" may
  not contain '0' at all, fill at fixed position instead.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42207 b2dd03c8-39d4-4d8f-98ff-823fe69b080e