demux_lavf: fix seeking in ogg audio streams

This detected the first packet demuxed after a seek as timestamp
discontinuity. Obviously this is non-sense. Since the OGG radio streams
for which this feature was introduced are normally unseekable, it's
simple to fix this: simply disable it (if in auto mode, the default) as
soon as a seek is performed. This code is never called if the stream is
considered unseekable, unless the user forced it.

There's still a chance this linearization is performed before a seek
happens. This will be a bit awkward, but no worse than without this
feature, since seeking with timestamp resets is inherently broken in
both mpv and libavformat.

Fixes: #6974
Fixes: 27fcd4d

demux/demux_lavf.c

@@ -1213,6 +1213,9 @@ static void demux_seek_lavf(demuxer_t *demuxer, double seek_pts, int flags)
                          "luck by enabling or enlarging the mpv cache.\n");
     }
 
+    if (priv->linearize_ts < 0)
+        priv->linearize_ts = 0;
+
     if (!(flags & SEEK_FORWARD))
         avsflags = AVSEEK_FLAG_BACKWARD;