Original commit message from CVS:
* ext/ffmpeg/gstffmpegdemux.c: (gst_ffmpegdemux_averror),
(gst_ffmpegdemux_init), (gst_ffmpegdemux_close),
(gst_ffmpegdemux_push_event), (gst_ffmpegdemux_send_event),
(gst_ffmpegdemux_src_query), (gst_ffmpegdemux_open),
(gst_ffmpegdemux_loop), (gst_ffmpegdemux_sink_activate),
(gst_ffmpegdemux_sink_activate_push),
(gst_ffmpegdemux_sink_activate_pull):
Don't try to use or remove non-existing pads.
Use _scale some more.
Don't try to do maths with invalid timestamps. Fixes #359545.
Prepare for push based scheduling.