Fix memory leak in assert scopes.

R=svenpanne@chromium.org
BUG=246567

Review URL: https://chromiumcodereview.appspot.com/15709020

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15106 ce2b1a6d-e550-0410-aec6-3dcde31c8c00