Buffer corruption in b64.c

[t0bias-r]

chunk_decode writes 3 bytes to pout in any case - even when nlen is less than 3.
This is a write beyond the allowed buffer.
see b64.c,160

        if (tot_sz >= nlen)
        {
            chunk_decode(buf, pout);
            tot_sz -= nlen; /* Take care of subtracting pad bytes. */
            pout += nlen;
        }