* gc.c (before_gc_sweep): don't optimize it to avoid segv on Ubuntu
  10.04 gcc 4.4.
  http://u32.rubyci.org/~chkbuild/ruby-trunk/log/20130527T190301Z.diff.html.gz

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41083 b2dd03c8-39d4-4d8f-98ff-823fe69b080e