[llvm] [MemProf] Add assert to make sure ContextIterBegin isn't at StackContext.end() (PR #81004)

[Teresa Johnson via llvm-commits]

https://github.com/teresajohnson requested changes to this pull request.

This assert will currently fire. See my comments on https://github.com/llvm/llvm-project/pull/78264

https://github.com/llvm/llvm-project/pull/81004