Proton C++ will crash if told to run 0 threads

If you call container::run(0) - i.e. tell it to run 0 threads it will crash.

I think the correct thing to do here is just ignore all values less than 1 and treat them as if they were 1.

Another possibility would be to treat 0 as a "default" value - and run a "good" number of threads for the circumstances.