Handle 0-size sync packets when only parsing headers.

Currently, the duration of those packets is just discarded
when enabling parsing, thus the output of the Metal Gear Solid
demuxer breaks completely when just setting AVSTREAM_PARSE_HEADERS.
The result will not be correct if a parser creates a delay even
with PARSER_FLAG_COMPLETE_FRAMES and there might be other cases
where it does not work correct, but just discarding them as it
is done currently seems worse.

Signed-off-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>

libavformat/utils.c

@@ -1133,6 +1133,9 @@ static int parse_packet(AVFormatContext *s, AVPacket *pkt, int stream_index)
         av_init_packet(&flush_pkt);
         pkt = &flush_pkt;
         got_output = 1;
+    } else if (!size && st->parser->flags & PARSER_FLAG_COMPLETE_FRAMES) {
+        // preserve 0-size sync packets
+        compute_pkt_fields(s, st, st->parser, pkt);
     }
 
     while (size > 0 || (pkt == &flush_pkt && got_output)) {