ext/ffmpeg/: Handle buffers with invalid duration correctly even in the case where we have a demuxer AND a decoder fr...

Original commit message from CVS:
2007-07-25  Julien MOUTTE  <julien@moutte.net>

* ext/ffmpeg/gstffmpegdec.c: (gst_ffmpegdec_video_frame):
* ext/ffmpeg/gstffmpegdemux.c: (gst_ffmpegdemux_loop): Handle
buffers with invalid duration correctly even in the case where
we have a demuxer AND a decoder from ffmpeg linked.
Fixes: #460274.