webrtc: Fix possible use-after-free of GstWebRTCICETransport

webrtc: Fix possible use-after-free of GstWebRTCICETransport

Because of the asynchronous resolving of mDNS ICE candidates it is
possible that GstWebRTCICE outlives webrtcbin. This in turn prolongs
the lifetime of the GstWebRTCNiceStream objects via refs in
nice_stream_map. Thus the GstWebRTCICETransport objects held in
GstWebRTCNiceStream may be invalid at the time they are accessed by
the _on_candidate_gathering_done() callback since GstWebRTCNiceStream
doesn't take a reference to them. Doing so would create a circular
reference, so instead this commit introduces weak references to the
transport objects and then we can check if the objects are valid before
accessing them.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/3502>