first overlap doesn't return expected results

I expected:

```
user=> (it/first-overlapping (ts [1 10] [12 16]) [19 40])
nil
```

but got
```
user=> (it/first-overlapping (ts [1 10] [12 16]) [19 40])
[12 16]
```

Forgive me if the output is correct, im still learning about interval tree as i work through these examples.