Added checks to ensure sizes fit into int before casting.

I made these changes to solve AppVeyor warnings from Building Domoticz.

The third parameter (len) is expected to be of type int in OpenSSL's function signature:
int BIO_write(BIO *b, const void *data, int dlen);

The Last Warning is from commit "71c3d36"
if (get_type() == json::type::number) return system_clock::from_time_t(std::round(as_number()));

But this is changed in the last master to;
if (get_type() == json::type::number) return date(std::chrono::seconds(std::llround(as_number())));

so this is probably already solved.

Kind Regards