Pad the environment with 0 rather than the receiver to avoid influencing the representation of the join.

R=titzer@chromium.org

Review URL: https://chromiumcodereview.appspot.com/23707055

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@16861 ce2b1a6d-e550-0410-aec6-3dcde31c8c00