fix: check the format of the event name

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 3b78117bf6

test/arraybuffer.js

@@ -50,7 +50,7 @@ describe('parser', function() {
   it('cleans itself up on close', function() {
     var packet = {
       type: parser.BINARY_EVENT,
-      data: [new ArrayBuffer(2), new ArrayBuffer(3)],
+      data: ["foo", new ArrayBuffer(2), new ArrayBuffer(3)],
       id: 0,
       nsp: '/'
     };