[clang] [analyzer] Hotfix a boolean conversion crash in the Z3 SMTConv (PR #158276)

[Donát Nagy via cfe-commits]

https://github.com/NagyDonat approved this pull request.

LGTM.

> I'd only highlight here that I think using the argument that anything is better than having a crash - while is true, still leaves a bitter taste.
I have not counted how many similar crashes we had within the Z3 conversion layer, but I still stand by what I've said, that we need to investigate these and put an end to this.

I agree that this leaves a bitter taste. I feel an urge to jump into this and clean up the situation after a through investigation, but there are too many issues where I feel this urge and I think some others are more relevant than this one. (E.g. now I'm working on generalizing the `security.ArrayBound` logic for other checkers that do bounds checking and I hope that it will provide tangible improvements.)

https://github.com/llvm/llvm-project/pull/158276