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

I'm using the LLVM C bindings through the llvm-sys rust crate, and
noticed that LLVMGetFirstDbgRecord and LLVMGetLastDbgRecord are
segfault-ing when called on instructions without debug markers. I found
out it's missing a null pointer check. This PR fixes the issue.

Author: arthaud

llvm/lib/IR/Core.cpp

@@ -2994,6 +2994,8 @@ LLVMValueRef LLVMIsATerminatorInst(LLVMValueRef Inst) {
 
 LLVMDbgRecordRef LLVMGetFirstDbgRecord(LLVMValueRef Inst) {
   Instruction *Instr = unwrap<Instruction>(Inst);
+  if (!Instr->DebugMarker)
+    return nullptr;
   auto I = Instr->DebugMarker->StoredDbgRecords.begin();
   if (I == Instr->DebugMarker->StoredDbgRecords.end())
     return nullptr;
@@ -3002,6 +3004,8 @@ LLVMDbgRecordRef LLVMGetFirstDbgRecord(LLVMValueRef Inst) {
 
 LLVMDbgRecordRef LLVMGetLastDbgRecord(LLVMValueRef Inst) {
   Instruction *Instr = unwrap<Instruction>(Inst);
+  if (!Instr->DebugMarker)
+    return nullptr;
   auto I = Instr->DebugMarker->StoredDbgRecords.rbegin();
   if (I == Instr->DebugMarker->StoredDbgRecords.rend())
     return nullptr;

llvm/tools/llvm-c-test/debuginfo.c

@@ -325,6 +325,13 @@ int llvm_test_dibuilder(void) {
   LLVMValueRef Phi2 = LLVMBuildPhi(Builder, I64, "p2");
   LLVMAddIncoming(Phi2, &Zero, &FooEntryBlock, 1);
 
+  // Test that LLVMGetFirstDbgRecord and LLVMGetLastDbgRecord return NULL for
+  // instructions without debug info.
+  LLVMDbgRecordRef Phi1FirstDbgRecord = LLVMGetFirstDbgRecord(Phi1);
+  assert(Phi1FirstDbgRecord == NULL);
+  LLVMDbgRecordRef Phi1LastDbgRecord = LLVMGetLastDbgRecord(Phi1);
+  assert(Phi1LastDbgRecord == NULL);
+
   // Insert a non-phi before the `ret` but not before the debug records to
   // test that works as expected.
   LLVMPositionBuilder(Builder, FooVarBlock, Ret);