[llvm] [NFCI] Check for non-null before dereferencing a VPBB ptr (PR #190403)

[Florian Hahn via llvm-commits]

fhahn wrote:

@calebwat it would be good if you would update your GitHub settings so a real email is visible and used for the commit. I think it is currently set to `<107081575+calebwat at users.noreply.github.com>` which means you won't get any email notifications in case there's any breakages in the post-commit bots

https://github.com/llvm/llvm-project/pull/190403