Assertion in windowing code about null window pos

[sakridge]

Seeing this assertion in a leader + client setup. Cuda enabled, erasure not enabled:
thread 'solana-broadcaster' panicked at 'assertion failed: win[pos].is_none()', src/streamer.rs:483:13

[sakridge]

This assert fires, so we are dequeuing a full window of blobs

    Crdt::index_blobs(crdt, &blobs, receive_index)?;
    // keep the cache of blobs that are broadcast
    {
        let mut win = window.write().unwrap();
        assert!(blobs.len() < win.len());  <<<<<<<======
        for b in &blobs {
            let ix = b.read().unwrap().get_index().expect("blob index");