[llvm] [llvm-c] Add missing nullptr check in LLVMGetFirstDbgRecord (PR #151101)

[Maxime Arthaud via llvm-commits]

arthaud wrote:

It looks like llvm/tools/llvm-c-test/debuginfo.c is testing functions that create LLVM IR, while my PR is fixing functions that read LLVM IR. We don't have C functions to create debug markers, from what I understand, so I'm not sure how to test this properly without having to add new C binding functions.

Do we have tests somewhere for reading a .bc or .ll file with the C bindings?

https://github.com/llvm/llvm-project/pull/151101