avformat/mxfdec: Check container_ul->desc before use

Fixes: CID1592939 Dereference after null check

Sponsored-by: Sovereign Tech Fund
Reviewed-by: Tomas Härdin <git@haerdin.se>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 4cab028bd0)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/mxfdec.c

@@ -3064,6 +3064,7 @@ static int mxf_parse_structural_metadata(MXFContext *mxf)
             if (container_ul->desc)
                 av_dict_set(&st->metadata, "data_type", container_ul->desc, 0);
             if (mxf->eia608_extract &&
+                container_ul->desc &&
                 !strcmp(container_ul->desc, "vbi_vanc_smpte_436M")) {
                 st->codecpar->codec_type = AVMEDIA_TYPE_SUBTITLE;
                 st->codecpar->codec_id = AV_CODEC_ID_EIA_608;