ext/ffmpeg/gstffmpegdemux.c: If the duration returned by ffmpeg is 0, it doesn't mean the file has no duration, but t...

Original commit message from CVS:
* ext/ffmpeg/gstffmpegdemux.c: (gst_ffmpegdemux_open):
If the duration returned by ffmpeg is 0, it doesn't mean the file has
no duration, but that it doesn't know the end.
Fixed accordingly for segment_stop.