Holding more than one threadsafe function causes SIGABRT on Drop

An example repo: https://github.com/pimeys/napi-bug-repo

Creating two classes, both holding a threadsafe callback function, will dump core and cause SIGABRT on exit.

```sh
❯ uname -a
Linux naunau 5.11.8 #1-NixOS SMP Sat Mar 20 09:51:14 UTC 2021 x86_64 GNU/Linux
```

```toml
[[package]]
name = "napi"
version = "1.3.3"
```