RFC4880_encode_count doesn't return consistent results when processing exact iterations

[evpo]

The function returns RFC4880 code + 1 when given exact iterations. The code below demonstrates this behavior:

using namespace Botan;
std::stringstream size_t decode_result = OpenPGP_S2K::decode_count(160U);
stm << " decode(160)=" << decode_result << '\n';
stm << " encode(" << decode_result << ")=" <<
    static_cast<unsigned>(OpenPGP_S2K::encode_count(decode_result)) << '\n';

Output:

decode(160)=1048576
encode(1048576)=161

This issue is related: evpo/EncryptPad#82

The behavior makes it impossible to open encrypted files.

PR is coming