libcore: Fix assertion failure in vec::windowe.

vec::windowed fails if given window size is greater than vector length + 1.

```rust
for vec::windowed(7, &[1,2,3,4,5,6]) |vs| { fail!(); } // => do nothing
for vec::windowed(8, &[1,2,3,4,5,6]) |vs| { fail!(); } // => assertion failure in vec::slice
```