Add guard in AboutToFinishCallback to prevent race condition when pipeline is being torn down. This prevents the callback from trying to set next URL while the pipeline is being destroyed, which caused crashes in GStreamer's decodebin3.
Fixes issue where rapidly switching tracks could cause segmentation fault in gst_decodebin_input_link_to_slot.
See: https://gitlab.freedesktop.org/gstreamer/gstreamer/-/issues/4626Fixes#1863
Co-Authored-By: Jonas Kvinge <jonas@jkvinge.net>
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