GStreamerWebRtcDataChannel::new panics in various cases

[jdm]

The expect in GStreamerWebRtcDataChannel::new causes panics if:

• the associated webrtc element is closed
• both DataChannelInit::max_packet_lifetime and DatachannelInit::max_retransmits are present
• DataChannelInit::negotiated is true and DataChannelInit::id is not present
• DataChannelInit::id is present and isn't valid

[jdm]

Another example of a panic, stemming from a gstreamer that isn't prepared to create webrtcbin elements: https://github.com/servo/servo/issues/28357