A specially crafted packet could be incorrectly decoded.

Example:

```js
const decoder = new Decoder();

decoder.on("decoded", (packet) => {
  console.log(packet.data); // prints [ 'hello', [Function: splice] ]
})

decoder.add('51-["hello",{"_placeholder":true,"num":"splice"}]');
decoder.add(Buffer.from("world"));
```

As usual, please remember not to trust user input.

Backported from b5d0cb7

Diff: 3.4.1...3.4.2

A packet like '2[{"toString":"foo"}]' was decoded as:

{
  type: EVENT,
  data: [ { "toString": "foo" } ]
}

Which would then throw an error when passed to the EventEmitter class:

> TypeError: Cannot convert object to primitive value
>    at Socket.emit (node:events:507:25)
>    at .../node_modules/socket.io/lib/socket.js:531:14

History of the isPayloadValid() method:

- added in [78f9fc2](78f9fc2) (v4.0.1, socket.io@3.0.0)
- updated in [1c220dd](1c220dd) (v4.0.4, socket.io@3.1.0)

Backported from 3b78117

Diff: 3.4.2...3.4.3