Another emonPi stopped logging thread

I have checked the other processes and I think this is the only one that is currently vulnerable to this so I have submitted a PR for consideration, but I do not think it is an ideal fix. It simply tries to avoid a crash/exception.

It is not ideal to be passing a value of zero to the next process, for example if that is a log to feed, the previous good “current” value could be over-written with 0 watts just because the input was duplicated (if buffered write is not in use, see Data Viewer questions - #14 by pb66).