Fix: Index fails to search first inserted vector

[miamia0]

I tried using the code below, but the search result is empty.
I attempted to fix the code in this PR to ensure it returns the correct result.

 let hnsw: Hnsw<f32, DistCosine> = Hnsw::new(15, 100_000, 20, 500_000, DistCosine {});
hnsw.insert((&[1.0, 0.0, 0.0, 0.0], 0));
let result = hnsw.search(&[1.0, 0.0, 0.0, 0.0], 2, 10);

[jean-pierreBoth]

Thanks for the bug. I fixed it somewhat differently