flvdec: fix lack of duration for some files

Fixes #4579
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/flvdec.c

@@ -847,7 +847,7 @@ static int flv_read_packet(AVFormatContext *s, AVPacket *pkt)
                 int type;
                 meta_pos = avio_tell(s->pb);
                 type = flv_read_metabody(s, next);
-                if (type == 0 && dts == 0 || type < 0) {
+                if (type == 0 && dts == 0 || type < 0 || type == TYPE_UNKNOWN) {
                     goto skip;
                 } else if (type == TYPE_ONTEXTDATA) {
                     avpriv_request_sample(s, "OnTextData packet");