[llvm] kaleidoscope: add missing check in the FunctionAST::codegen (PR #76322)

[via llvm-commits]

TiborGY wrote:

> @TiborGY, yes I think, having that will keep the tutorial and the code in the repo in sync

Great, could you please rebase your branch on top of main and confirm that this example still builds and runs?
If yes, than I have no objections to having this merged.

https://github.com/llvm/llvm-project/pull/76322