[BCSharp]

Bug: code compiled for tracing has its frames pushed and popped on the frame stack by the thread's trace listener, on the FrameEnter and FrameExit events. A thread without a trace function got a (dummy) listener only while some thread had tracing on, so when tracing was switched on or off between the two events of a frame, only one of them was handled:

• A traced function that called sys.settrace(None) left its frame on the stack for good, as did its traced callers.
• A frame entered while no thread was tracing and exited after another thread switched tracing on popped a frame it had not pushed: for exec'd code, the frame that exec pushes for the same code object, so exec then popped its caller's frame. At exit this could fail with "Index was out of range" in RunnableScriptCode.PopFrame.

The dispatcher now registers a dummy listener for any thread that runs code compiled for tracing, whether or not tracing is on at the moment. A dummy listener only maintains the frame stack and no longer calls trace functions: as in CPython, a thread without a trace function gets no trace events, not even for frames that have a local trace function.

[slozier]

Thanks! Looks good to me!