mxfdec: fix NULL pointer dereference

Metadata streams have priv_data set to NULL.
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>

libavformat/mxfdec.c

@@ -391,7 +391,7 @@ static int mxf_get_stream_index(AVFormatContext *s, KLVPacket *klv)
     for (i = 0; i < s->nb_streams; i++) {
         MXFTrack *track = s->streams[i]->priv_data;
         /* SMPTE 379M 7.3 */
-        if (!memcmp(klv->key + sizeof(mxf_essence_element_key), track->track_number, sizeof(track->track_number)))
+        if (track && !memcmp(klv->key + sizeof(mxf_essence_element_key), track->track_number, sizeof(track->track_number)))
             return i;
     }
     /* return 0 if only one stream, for OP Atom files with 0 as track number */