[llvm] [NFC]Add assert to avoid possibly deref nullptr (PR #65564)

[David Blaikie via llvm-commits]

dwblaikie wrote:

(doesn't seem to be debug info related)

+1 to @aprantl and @JDevlieghere - if the null deref was reachable, then this needs proper error handling, the assert is insufficient/not the right tool. (& this needs a test case, if it is reachable)

https://github.com/llvm/llvm-project/pull/65564