Fix OrderedHashTabelIterator accessors.

They might be undefined for uninitialized iterators.
The rest of the code is ready for this eventuality.

R=arv@chromium.org, adamk@chromium.org
BUG=403292
LOG=N

Review URL: https://codereview.chromium.org/468813003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23126 ce2b1a6d-e550-0410-aec6-3dcde31c8c00