CopyWords cannot actually copy zero words---it'd clobber destiantion with
the first word of source.

Add an ASSERT to check this condition plus update array builtins to verify
for amount of copied data when necessary.

TBR=vitalyr@chromium.org

Review URL: http://codereview.chromium.org/1559004

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@4313 ce2b1a6d-e550-0410-aec6-3dcde31c8c00