C++: Check for negative Heartbeat Wakeup Times

This checks if the calculated time to check for the next heartbeat is in the future.

I'm not sure if this is the optimal solution, but using std::min(wakeup, remote.last_checked + remote.interval) would mean that we disregard that heartbeats (or extrasystoles) might have arrived earlier than the maximum interval.

This closes #112 (closed)