mxfdec: fix NULL pointer dereference in mxf_read_packet_old

Metadata streams have priv_data set to NULL.
Reviewed-by: Josh de Kock <josh@itanimul.li>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>

libavformat/mxfdec.c

@@ -3135,7 +3135,7 @@ static int mxf_read_packet_old(AVFormatContext *s, AVPacket *pkt)
                 if (mxf->nb_index_tables >= 1 && mxf->current_edit_unit < t->nb_ptses) {
                     pkt->dts = mxf->current_edit_unit + t->first_dts;
                     pkt->pts = t->ptses[mxf->current_edit_unit];
-                } else if (track->intra_only) {
+                } else if (track && track->intra_only) {
                     /* intra-only -> PTS = EditUnit.
                      * let utils.c figure out DTS since it can be < PTS if low_delay = 0 (Sony IMX30) */
                     pkt->pts = mxf->current_edit_unit;