Fix bug in keyed load stub for strings.
Fix bug in keyed load stub for strings.

Instead of returning the empty string when indexing
a string out of bounds we now correctly return undefined.

Review URL: http://codereview.chromium.org/542089

git-svn-id: b158db1e4b4ab85d4c9e510fdef4b1e8c614b15b@3611 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh