Prevent empty handle dereference in Runtime_InternalNumberFormat.

R=jochen@chromium.org
BUG=chromium:275467

Review URL: https://codereview.chromium.org/22859025

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16220 ce2b1a6d-e550-0410-aec6-3dcde31c8c00