Turn assert into runtime assertion to make fuzzer happy

R=mstarzinger@chromium.org
BUG=265369

Review URL: https://codereview.chromium.org/22284009

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16076 ce2b1a6d-e550-0410-aec6-3dcde31c8c00