hash.c: fix memory leak

* hash.c (ruby_setenv): fix memory leak on Windows, free
  environment strings block after check for the size.
  [ruby-dev:48323] [Bug #9977]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46550 b2dd03c8-39d4-4d8f-98ff-823fe69b080e