avoid divide by zero errors
avoid divide by zero errors

see https://community.openenergymonitor.org/t/another-emonpi-stopped-logging-thread/7267/8?u=pb66
This only avoids a crash, it's not ideal to return "power = 0" to the next process.
emoncms/emoncms,chaveiro/emoncms,inverse/emoncms,inverse/emoncms,chaveiro/emoncms,inverse/emoncms,emoncms/emoncms,inverse/emoncms,chaveiro/emoncms,emoncms/emoncms,emoncms/emoncms,emoncms/emoncms,chaveiro/emoncms