Index out of bounds on Vector::iter()

[krobelus]

Below is a program that causes a crash.
The problem seems to have been introduced in 6d0b068.

fn main() {
    let mut x = im::Vector::new();
    for _ in 0..262 {
        x.push_back(0);
    }
    for _ in 0..97 {
        x.pop_front();
    }
    for &offset in &[160, 163, 160] {
        x.remove(offset);
    }
    for _ in 0..5 {
        x.pop_front();
    }
    for _ in 0..128 {
        x.push_back(0);
    }
    x.push_back(0);
    for _ in x.iter() {
    }
}