time_t NTPClient::get_timestamp() incorrect error logic

[gmgallo]

The problem:
time_t is defined as:

typedef unsigned int time_t; /* date/time in unix secs past 1-Jan-70 */

therefore, get_timestamp() can't return a negative value on error.
Instead, it should return 0 on error or be changed as follows to be consistent with error report in other Mbed OS modules:

int NTPClient::get_timestamp(time_t *time, int timeout) 
{
    const time_t TIME1970 = (time_t)2208988800UL;
    int ntp_send_values[12] = {0};
    int ntp_recv_values[12] = {0};

    SocketAddress nist;

    if (iface) 
    {
        int ret_gethostbyname = iface->gethostbyname(nist_server_address, &nist);

        if (ret_gethostbyname < 0) {
            // Network error on DNS lookup
            return ret_gethostbyname;
        }

        nist.set_port(nist_server_port);

        memset(ntp_send_values, 0x00, sizeof(ntp_send_values));
        ntp_send_values[0] = '\x1b';

        memset(ntp_recv_values, 0x00, sizeof(ntp_recv_values));

        UDPSocket sock;
        sock.open(iface);
        sock.set_timeout(timeout);

        sock.sendto(nist, (void*)ntp_send_values, sizeof(ntp_send_values));

        SocketAddress source;
        const int n = sock.recvfrom(&source, (void*)ntp_recv_values, sizeof(ntp_recv_values));

        if (n > 10) /* success */
        {
            *time = ntohl(ntp_recv_values[10]) - TIME1970;

            return NSAPI_ERROR_OK; /* return 0 if no error */

        } else {
            if (n < 0) 
            {
                // Network error
                return n;

            } else {
                // No or partial data returned
                return -1;
            }
        }

    } else {
        // No network interface
        return -2;
    }
}

[stephwongwt]

We're also facing this issue in our project and is currently implementing a version of #18 to resolve this issue.