unsigned int avpriv_toupper4(unsigned int x)
{
    return av_toupper(x & 0xFF) +
          (av_toupper((x >>  8) & 0xFF) << 8)  +
          (av_toupper((x >> 16) & 0xFF) << 16) +
((unsigned)av_toupper((x >> 24) & 0xFF) << 24);
}

The product writes data past the end, or before the beginning, of the intended buffer.