OS::MemMove/OS::MemCopy: Don't call through to generated code when size == 0 to avoid prefetching invalid memory

BUG=chromium:233500

Review URL: https://codereview.chromium.org/14365011

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@14361 ce2b1a6d-e550-0410-aec6-3dcde31c8c00