Bug in DoublePriorityQueue `iter_mut`

[Takashiidobe]

It looks like DoublePriorityQueue panics when calling next_back immediately on an iter_mut call to a DPQ.

use priority_queue::DoublePriorityQueue;

#[test]
fn iter_mut_next_back_panics_on_first_call() {
    let mut pq = DoublePriorityQueue::new();
    pq.push("apples", 1);
    pq.push("bananas", 2);

    let mut iter = pq.iter_mut();
    iter.next_back();
}

Running that test with RUST_BACKTRACE=1 cargo test --test dpq_iter_mut_next_back -- --no-capture:

thread 'iter_mut_next_back_panics_on_first_call' panicked at /home/takashi/current/priority-queue/src/double_priority_queue/iterators.rs:198:9:
attempt to subtract with overflow

That line is here.

Creation of iter_mut is here?

My guess is that pos is supposed to be at the end of the DPQ, but new creates it at the start, and then the next_back code runs which immediately subtracts with underflow. I think just having a front and back position and using those to keep state should suffice?