ext/ffmpeg/gstffmpegmux.c: Only set the mux->opened flag after we've successfully written the header. This way we don...

Original commit message from CVS:
* ext/ffmpeg/gstffmpegmux.c: (gst_ffmpegmux_request_new_pad),
(gst_ffmpegmux_setcaps), (gst_ffmpegmux_collected):
Only set the mux->opened flag after we've successfully written the
header. This way we don't crash in mysterious ways if we can't write
the header for some reason (e.g. due to having accepted caps the
format doesn't really allow), then return a GST_FLOW_ERROR, and
then still receive another buffer afterwards despite having previously
returned FLOW_ERROR (#403168).
Also some minor logging improvements.