NTPClient: ignore invalid timestamps

[fooker]

This fixes #2040 by checking against 0 timestamps.

[slaff]

I like the "reflowing" of the code. Can you please change this original comment // Got a dodgy packet so treat it as we would a response failure to // Got a dodgy packet - ignore it and retry ?

[slaff]

@fooker

Some things to fix before we can merge this PR

1. Rebase your PR against the lastest develop branch.
2. Fix the coding style:

@@ -143,7 +143,7 @@ void NtpClient::onReceive(pbuf* buf, IpAddress remoteIP, uint16_t remotePort)
 	pbuf_copy_partial(buf, &timestamp, sizeof(timestamp), 40); // Copy only timestamp.
 	timestamp = ntohl(timestamp);
 
-	if (timestamp == 0) {
+	if(timestamp == 0) {
 		// Got a invalid timestamp - retry
 		startTimer(NTP_RESPONSE_TIMEOUT_MS);
 		return;'

You can run:

cd $SMING_HOME; make cs

3. Fix any issue reported by travis, codacy and appveyor.

[mikee47]

All good. I do have a suggestion for more self-documenting code:

auto retry = [this]() { startTimer(NTP_RESPONSE_TIMEOUT_MS); };

if(mode != NTP_MODE_SERVER) {
    return retry();
}

if(ver != NTP_VERSION && ver != (NTP_VERSION - 1)) {
    return retry();
}

...

if(timestamp == 0) {
    return retry();
}

[mikee47]

@fooker Don't worry about appveyor - rebase will fix that.

[fooker]

@mikee47 I really like the idea using a lambda for code deduplication. I've adopted it.
@slaff rebased to develop and fixed formatting (by implementing suggestion from above).