not to return functions that have not been loaded.

This fixes crashes on expedia.com:

  http://code.google.com/p/chromium/issues/detail?id=4526
Review URL: http://codereview.chromium.org/11272

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@799 ce2b1a6d-e550-0410-aec6-3dcde31c8c00