Adding Validity-Checks for incoming packets

[CJuette]

Using the Library with Wifi I experienced some problems with timestamps occasionally being way off, by that I mean more than 1,000,000,000 s off.

After some investigation I discovered that the board had captured some other UDP-Packets and had interpreted them as NTP-Packets. Somehow the application PowerFolder had sent them to Port 1337 of the board (why that is is another problem).

However I decided to implement a few validity checks before actually interpreting the packets as NTP-Packets. Feel free to edit my implementation. I implemented a method "isValid()" which gets called in "forceUpdate".

[mmmilan]

I have the same problem, sometimes I get 2036-02-07 07:28:... as the current time, which means secsSince1900 is all zeroes.
this->_udp->read(this->_packetBuffer, NTP_PACKET_SIZE) call in forceUpdate() should be checked to if it returns NTP_PACKET_SIZE.

[CLAassistant]

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