Add support for channelsToTreat to be empty

It's the case when providing a single channel in `channels.txt` for
instance.

main.cpp

@@ -712,7 +712,7 @@ void markChannelAsRequiringTreatmentIfNeeded(unsigned short threadId, string cha
     if(channelsAlreadyTreated.find(channelId) == channelsAlreadyTreated.end() && channelsToTreatRev.find(channelId) == channelsToTreatRev.end())
     {
         // It is unclear to me why `channelsToTreat.end()->first + 1` doesn't work here.
-        unsigned int channelsToTreatIndex = channelsToTreat.rbegin()->first + 1;
+        unsigned int channelsToTreatIndex = !channelsToTreat.empty() ? channelsToTreat.rbegin()->first + 1 : channelsAlreadyTreated.size();
         channelsToTreat[channelsToTreatIndex] = channelId;
         channelsToTreatRev[channelId] = channelsToTreatIndex;