Crash when doing data propagation with a circuit in the network

[chateauferret]

Qt Nodes is fine with a circuit in the network if there is no data propagation through the nodes, but if there is, it causes an infinite loop and a stack overflow which leads to a crash.

I have some code for finding such loops somewhere (from an attempt of my own years ago), I can try and adapt that. Alternatively we can start propagating the data and remember the connection that triggered it, then send the user a snotty message if we end up back where we started. That way would only find circuits in a propagating network, of course.