Obey VisualStudio checked iterator levels, and NULL checked iterators

When compiling under VisualStudio we need to first determine if checked iterators are enabled ( _ITERATOR_DEBUG_LEVEL ). We don't want to use the NDEBUG key, as we could be inside a project that is in Debug mode with disabled checked iterators.

Secondly if they are enabled we need to handle the use case of NULL iterators that get advanced by length zero. This last case is valid, but isn't supported by the checked iterators so we need to work around it.