ntploggps: Resolve issue #730 by ensuring relevant fields valid.
gpsd appears to be sending SKY sentences without satellites. gpsd also appears to be (sometimes) 'uSat' which has the number. So, grab 'device', 'tdop' and (the sum of used satellites or 'uSat'). Check that non of them are invalid, then package them for use. Also, address potential issue where on multi-device systems, another device can steal the thunder of the reporting device. That however, can never occur.