mxfdec: drop unneeded extradata padding

Found-by: Tomas Härdin <tomas.hardin@codemill.se>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/mxfdec.c

@@ -870,7 +870,7 @@ static int mxf_read_generic_descriptor(void *arg, AVIOContext *pb, int tag, int
                 av_log(NULL, AV_LOG_WARNING, "Duplicate sony_mpeg4_extradata\n");
             av_free(descriptor->extradata);
             descriptor->extradata_size = 0;
-            descriptor->extradata = av_malloc(size + FF_INPUT_BUFFER_PADDING_SIZE);
+            descriptor->extradata = av_malloc(size);
             if (!descriptor->extradata)
                 return AVERROR(ENOMEM);
             descriptor->extradata_size = size;