Fix copying extradata to codec in mxfdec.c

The code that copies any extradata from the MXFDescriptor to the codec does
not set the size, which it should otherwise the copied data is useless.
Attached it a patch to correct this.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/mxfdec.c

@@ -1601,8 +1601,10 @@ 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) {
             ff_generate_avci_extradata(st);
         }