bug fix for Full_RF12demo: packets with bad CRC were not being rejected

The full RF12demo firmware was allowing packets with bad CRCs to be printed out, causing them to be picked up by emoncms.

As emoncms would interpret them in some cases as values for new inputs, it would end up creating hundreds of inputs bringing the web UI to its knees (as well as bloating timestore/DB).

This fixes it so packets with bad CRCs are ignored again.