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