Experimental fix for ffi_closure_free causes dumping core/SEGV

http://u64.rubyci.org/~chkbuild/ruby-trunk/log/20130411T041301Z.log.html.gz#test%2F-ext-

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40233 b2dd03c8-39d4-4d8f-98ff-823fe69b080e