I'm not massively great at C but would this be a fix?

    void*
    memmove(void *vdst, void *vsrc, int n)
    {
        char *dst, *src;

        dst = vdst;
        src = vsrc;
        if(*vdst > *vsrc) {
            *vdst += n;
            *vsrc += n;
            while(n-- > 0)
                *dst-- = *src--;
        }
        else if(*vdst < *vsrc)
            while(n-- > 0)
                *dst++ == *src++;
        return vdst;
    }