getNtpTime() doesn't handle zero from NTP server response

[Anton-V-K]

The code of getNtpTime() in https://github.com/PaulStoffregen/Time/blob/master/examples/TimeNTP/TimeNTP.ino doesn't check the correctness of UDP packet's content (obtained from NTP server), and can potentially set invalid date (in future - 07-Feb-2036).
I'm referring to this code fragment:

if (size >= NTP_PACKET_SIZE) {
      Serial.println("Receive NTP Response");
      Udp.read(packetBuffer, NTP_PACKET_SIZE);  // read packet into the buffer
      unsigned long secsSince1900;
      // convert four bytes starting at location 40 to a long integer
      secsSince1900 =  (unsigned long)packetBuffer[40] << 24;
      secsSince1900 |= (unsigned long)packetBuffer[41] << 16;
      secsSince1900 |= (unsigned long)packetBuffer[42] << 8;
      secsSince1900 |= (unsigned long)packetBuffer[43];
      return secsSince1900 - 2208988800UL + timeZone * SECS_PER_HOUR;
}

Here secsSince1900 can be zero, when in certain cases ( like "rate limiting" - mentioned in arduino-libraries/NTPClient#84 ) NTP responds with zero time.

[gednz]

I am having this issue too