oggdec: Fix incorrect assumption about header/data interleaving

Currently (since the data_offset fix) the ogg demuxer assumes that
after the first non-header packets in any stream no more header packets
will follow.
This is not guaranteed, so change the code back again to wait until it
has finished the headers for all streams before returning from ogg_get_headers.

This fixes issue 2428.
Signed-off-by: Mans Rullgard <mans@mansr.com>

libavformat/oggdec.c

@@ -374,20 +374,20 @@ ogg_packet (AVFormatContext * s, int *str, int *dstart, int *dsize, int64_t *fpo
             os->segp = segp;
             os->psize = psize;
 
-            // We have reached the first non-header packet. All header
-            // packets must be complete before the first non-header
-            // one, so everything that follows must be non-header.
+            // We have reached the first non-header packet in this stream.
+            // Unfortunately more header packets may still follow for others,
+            // so we reset this later unless we are done with the headers
+            // for all streams.
             ogg->headers = 1;
 
             // Update the header state for all streams and
             // compute the data_offset.
-            s->data_offset = os->sync_pos;
+            if (!s->data_offset)
+                s->data_offset = os->sync_pos;
             for (i = 0; i < ogg->nstreams; i++) {
                 struct ogg_stream *cur_os = ogg->streams + i;
-                // Set stream header state to 0 if its last packet
-                // was a header.
                 if (cur_os->header > 0)
-                    cur_os->header = 0;
+                    ogg->headers = 0;
 
                 // if we have a partial non-header packet, its start is
                 // obviously at or after the data start