libcore: fix overflow/underflow in range_step

`uint::range_step` or `int::range_step` causes overflow or underflow as following.
code:
```rust
fn main() {
    for uint::range_step(3, 0, -2) |n| {
        println(fmt!("%u", n));
    }
}
```
output:
```
3
1
18446744073709551615
18446744073709551613
...
```
This commit fixes this behavior as follows.
```
3
1
```