Add handling for NTP server returning 0

Fixes #193, fixes 84, fixes
Credits to @frankB415 for the Code in Issue #193!

Author: Der-Schubi

Diff for: NTPClient.cpp

@@ -112,6 +112,10 @@ bool NTPClient::forceUpdate() {
   // this is NTP time (seconds since Jan 1 1900):
   unsigned long secsSince1900 = highWord << 16 | lowWord;
 
+  if (secsSince1900 == 0 ) {
+    return false;
+  }
+
   this->_currentEpoc = secsSince1900 - SEVENZYYEARS;
 
   return true;  // return true after successful update