Code review comment for lp:~afrantzis/mir/fix-1189770

> termination_exception is accessed in run_mir() without locking the mutex.

I was thinking that our internal threads would have either not started or finished in the two points we access termination_exception in run_mir(). I guess an external user could be using mir::terminate_with_exception(), although I can't think of a valid use case for it. In any case, fixed.