Fix potential infinite discard loop.

Fixes trac issue #438.
Seeking in that sample would cause ogg_read_timestamp to fail
because ogg_packet would go into a state where all packets
of stream 1 would be discarded until the end of the stream.
Signed-off-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>

libavformat/oggdec.c

@@ -282,6 +282,9 @@ static int ogg_read_page(AVFormatContext *s, int *str)
 
     if (flags & OGG_FLAG_CONT || os->incomplete){
         if (!os->psize){
+            // If this is the very first segment we started
+            // playback in the middle of a continuation packet.
+            // Discard it since we missed the start of it.
             while (os->segp < os->nsegs){
                 int seg = os->segments[os->segp++];
                 os->pstart += seg;
@@ -368,7 +371,11 @@ static int ogg_packet(AVFormatContext *s, int *str, int *dstart, int *dsize,
 
         if (!complete && os->segp == os->nsegs){
             ogg->curidx = -1;
-            os->incomplete = 1;
+            // Do not set incomplete for empty packets.
+            // Together with the code in ogg_read_page
+            // that discards all continuation of empty packets
+            // we would get an infinite loop.
+            os->incomplete = !!os->psize;
         }
     }while (!complete);