Fix crasher in Object.getOwnPropertySymbols

R=arv@chromium.org, mstarzinger@chromium.org
BUG=346141
LOG=Y

Review URL: https://codereview.chromium.org/177883002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19539 ce2b1a6d-e550-0410-aec6-3dcde31c8c00