mxfdec: set extradata size

The line setting it was mistakenly removed in
ecf442a58b

libavformat/mxfdec.c

@@ -1556,8 +1556,10 @@ static int mxf_parse_structural_metadata(MXFContext *mxf)
         }
         if (descriptor->extradata) {
             st->codec->extradata = av_mallocz(descriptor->extradata_size + FF_INPUT_BUFFER_PADDING_SIZE);
-            if (st->codec->extradata)
+            if (st->codec->extradata) {
                 memcpy(st->codec->extradata, descriptor->extradata, descriptor->extradata_size);
+                st->codec->extradata_size = descriptor->extradata_size;
+            }
         } else if (st->codec->codec_id == AV_CODEC_ID_H264) {
             ret = ff_generate_avci_extradata(st);
             if (ret < 0)