"undefined" doesn't mean it has no state. it does have some state. it's just that this state can't be predicted. it can still be recorded and used in debugging backwards.

2 comments about that: First, it must be well defined in order for the operating system to be able to do something about the process. It may not be consistent, predictable to userspace, or runnable, but it must be something possibly sensible. Therefore, it doesn't matter, the debugger can still inspect it, and the programmer can make sense of this state.

Second, the debugger generally catches the signal before it gets handled.