fix: make OnlineSourceDebugInfo>>#readVariableNamed:fromContext: work…

[theseion]

… with dead contexts

Use #currentPC instead of pc to access the PC of the context to prevent errors on dead contexts, where pc is nil.

fixes #18718

[Ducasse]

Tx Max.
When do you have dead contexts from which you want to access variables.
Because we were thinking about it when we worked on the debug info and revise the complete debug API.

[theseion]

Fuel, obviously ;) When you deserialize a context, it will be dead.

Ignore that. I have a test in Fuel that creates a dead context, serializes and deserializes it, and then reads the value of a temporary variable. This used to work but no longer does. It's not a big issue and I could rewrite the test to use a live context instead.

[Ducasse]

I think that we can integrate and ask guille to review later.