[lldb] Add checks before dereferencing (NFC)

[kastiglione]

Fix a potential null dereference identified by @adrian-prantl, and also check the target too before dereferencing.

rdar://74371588

[kastiglione]

@swift-ci test

[kastiglione]

@adrian-prantl Is it worth keeping the check on the target? If there's a valid exe_scope, is a target guaranteed? If a target is guaranteed, maybe the return type of CalculateTarget should be Target &. EDIT: I see now it can't be Thread &.

[adrian-prantl]

@adrian-prantl Is it worth keeping the check on the target? If there's a valid exe_scope, is a target guaranteed? If a target is guaranteed, maybe the return type of CalculateTarget should be Target &. EDIT: I see now it can't be Thread &.

My understanding is that an ExecutionScope implies a Target. But — even if that's the case now, I'm strongly leaning towards checking everything that can be null because this may not be true in the future. In short: If it's guaranteed, the API should a reference. If it returns a pointer, we must check it.

[kastiglione]

If it returns a pointer, we must check it.

agreed