The c++ implementation of BSON returns Uncaught Assertion failed from c++ code instead of a proper catchable node js error.

The problem happens in case you are trying to serialize an object with the key name `toString` using serialize method.

Way to replicate the bug:

test.js

```
const BSON = require('bson-ext');

BSON.serialize({
  toString: {},
});

```

Run the code

`node test.js`

The process crashed with output
```
Assertion failed: (!h.IsEmpty()), function Unmaybe, file ../src/bson.h, line 60.
```

see https://github.com/mongodb-js/bson-ext/blob/4.0/src/bson.h#L60

It is not possible to catch this error in node js. In our case, it crashes our API server and takes some time to debug this issue.