Make sure the callback is reset when tearing down the PipeWireSession and that there is no concurrent access to it, which can potentially lead to a crash. Bug: webrtc:15386 Change-Id: I0b09002fe0479dc1cd946c80684bcc5d8754d54a Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/311546 Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org> Commit-Queue: Jan Grulich <grulja@gmail.com> Cr-Commit-Position: refs/heads/main@{#40464}