OGG demuxer: ensure that there's pages to read for duration calculation in

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@18523 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

libavformat/oggdec.c

@@ -450,7 +450,7 @@ ogg_get_length (AVFormatContext * s)
     size = url_fsize(s->pb);
     if(size < 0)
         return 0;
-    end = size > MAX_PAGE_SIZE? size - MAX_PAGE_SIZE: size;
+    end = size > MAX_PAGE_SIZE? size - MAX_PAGE_SIZE: 0;
 
     ogg_save (s);
     url_fseek (s->pb, end, SEEK_SET);