AddICECandidate might not verify ufrag

[jech]

Quoting https://w3c.github.io/webrtc-pc/#dom-peerconnection-addicecandidate:

5. If candidate.usernameFragment is not null, and is not equal to any username fragment present in the corresponding media description of an applied remote description, return a promise rejected with a newly created OperationError.

However, I cannot find the place in the Pion code where we check that that ufrag matches the SDP, so I suspect that we're wrongly accepting candidates from the wrong generation.