Fix the function-return detection

[pdigiglio]

By running the interpreter on Valgrind, I found out there is a bug in DBInterpreter::processReturn(). The top of the call stack is accessed when the stack is empty. This has undefined behavior.

Also, I think the call stack should be handled by the EventGenerator so that a call ID is pushed on a new CallEvent and popped on a ReturnEvent.

Maybe a segment stack would make return detection easier?

[pdigiglio]

• The segment-ID stack was not needed. The DBInterpreter now only keeps track of the last segment ID, which is enough to detect a function return;
• The CallStack is now handled by the EventGenerator as described above;
• There are no longer invalid accesses in the DBInterpreter::processReturn() member function.

[pdigiglio]

I still can't figure out how to properly detect function returns. Maybe there should be a call and segment stack per thread.