- Added special cutouts if a Vector has NULL data, which will now happen
  if an external string's resource has been deleted.
- Added an verification phase before old gen GC to verify that all real
  entries in the SymbolTable are valid symbols.
- Added test that verifies the correct behaviour of the workaround.

Review URL: http://codereview.chromium.org/66011

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@1689 ce2b1a6d-e550-0410-aec6-3dcde31c8c00