HDWallet: Use some kind of secure zero instead of memset that could be optimised out

[vincenthz]

• investigate the right construction: potential candidate already suggested
• make sure it's portable to windows

[nalyd88]

How about something like this?

void clear(void *buf, uint32_t const sz)
{
#ifdef _WIN32
    SecureZeroMemory(buf, sz);
#elseif  __STDC_LIB_EXT1__
    memset_s(buf, sz, 0, sz);
#else
    volatile unsigned char *volatile pnt_ =
        (volatile unsigned char *volatile) buf;
    size_t i = (size_t) 0U;
    while (i < sz)
        pnt_[i++] = 0U;
#endif
}

References
www.cryptologie.net
libsodium