Panic will appear when running this demo.

Panic will appear when running this demo.
```
package main

import (
"math/rand"
"sync"
"time"

"github.com/coder/hnsw"
)

var (
idToEmb = map[int64][]float32{
8832462: {0.16308737, -0.015082314, -0.13542788, 0.0050275815},
8836285: {0.12513474, -0.01645081, -0.18823123, 0.0038108851},
8833448: {0.12430125, -0.028154016, -0.16484132, -0.013979792},
8835979: {0.11317906, -0.015129599, -0.081378885, 0.14409661},
8833623: {0.047779884, -0.01809181, -0.11893867, -0.042758342},
8836020: {0.16370028, -0.018596498, -0.13393748, -0.025386846},
8833095: {0.13081397, -0.011383214, -0.043572865, 0.007917681},
105126: {0.087190986, -0.013881853, -0.16933066, 0.09613177},
}
)

func test2() {
g := &hnsw.Graph[int64]{
Distance: hnsw.CosineDistance,
Rng: rand.New(rand.NewSource(time.Now().UnixNano())),
M: 32,
Ml: 0.28,
EfSearch: 300,
}
wg := sync.WaitGroup{}
mtx := sync.Mutex{}
for id, emb := range idToEmb {
wg.Add(1)
go func(id int64, emb []float32) {
mtx.Lock()
g.Add(hnsw.MakeNode(id, emb))
mtx.Unlock()
wg.Done()
}(id, emb)
}
wg.Wait()

for id, emb := range idToEmb {
g.Add(hnsw.MakeNode(id, emb))
}

}

func main() {
test2()
}

```

There should be something wrong here.
```
if elevator != nil {
searchPoint = layer.nodes[*elevator]
}
```

Start by running the Go demo from the issue and reproducing the panic. Then inspect the `elevator` lookup around `searchPoint = layer.nodes[*elevator]`; done means the shown demo completes without panicking and the cause at that lookup is addressed.

Written by the indexing model from the issue text.