std::vec: Use a valid value as lifetime dummy in iterator

The current implementation uses `&v[0]` for the lifetime struct field,
but that is a dangling pointer for iterators derived from zero-length
slices.

Example:

    let v: [int, ..0] = [];  println!("{:?}", v.iter())

    std::vec::VecIterator<,int>{ptr: (0x7f3768626100 as *()), end: (0x7f3768626100 as *()), lifetime: &139875951207128}

To replace this parameter, use a field of type `Option<&'self ()>`
that is simply initialized with `None`, but still allows the iterator to
have a lifetime parameter.