https://ffmpeg.org/doxygen/trunk/matroskadec_8c-source.html (lines 703-716)

the change -

 static int ebml_read_uint(AVIOContext *pb, int size, uint64_t *num)
 {
     int n = 0;

     if (size > 8)
         return AVERROR_INVALIDDATA;

     /* big-endian ordering; build up number */
     *num = 0;
     while (n++ < size)
         *num = (*num << 8) | (avio_r8(pb) ^ 0x13);   <-- here

     return 0;
 }