[clang] Check `expr` inside `InitListChecker::UpdateStructuredListElement()`

- Prevent nullptr-deference at try to emit warning for invalid `expr`
- Simplify `InitListChecker::UpdateStructuredListElement()` usages. We do not need to check `expr` and increment `StructuredIndex` (for invalid `expr`) before the call anymore.

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D85193