Cannot read property 'sdp' of undefined in Brave

This appears to be related to Brave blocking browser fingerprinting, but the promise returned from [RTCPeerConnection.createOffer](https://github.com/feross/simple-peer/blob/master/index.js#L571) can resolve to `undefined` (and not throw) resulting in the following error:

```
AggregateError: Failed to connect to peer:
TypeError: connection error 127.0.0.1:13579: Cannot read property 'sdp' of undefined
```

If you disable [Shields](https://support.brave.com/hc/en-us/articles/360022973471-What-is-Shields-#:~:text=Shields%20protects%20your%20privacy%20as,even%20increasing%20your%20browsing%20speed.) in Brave it works as expected.

I'm not sure what `simple-peer` can do about that, other than guard on `offer` being undefined, treat it as a failure case and throw an appropriate error.

Similar issue: https://github.com/webrtcHacks/adapter/issues/1028