Qt: Fix RTP Player crash
marker_stream_ might not exist when playFinished is called. In particular, deleting the RtpAudioStreams might cause playFinished to be called when the audio streams delete their audio output in certain cases, such as if they are paused. (The behavior might be slightly different on Qt6, where QAudioSink is used vs. Qt5 where QAudioOutput is used.)
Fix #19596 (closed)