[llvm] [SelectionDAG] Fix null pointer dereference in resolveDanglingDebugInfo (PR #173500)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `llvm-clang-aarch64-darwin` running on `doug-worker-5` while building `llvm` at step 6 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/190/builds/33351

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 6 (test-build-unified-tree-check-all) failure: test (failure)
******************** TEST 'LLVM :: CodeGen/Generic/selectiondag-dbgvalue-null-crash.ll' FAILED ********************
Exit Code: -11

Command Output (stdout):
--
# RUN: at line 1
/Volumes/ExternalSSD/buildbot-root/aarch64-darwin/build/bin/llc -O3 < /Users/buildbot/buildbot-root2/aarch64-darwin/llvm-project/llvm/test/CodeGen/Generic/selectiondag-dbgvalue-null-crash.ll
# executed command: /Volumes/ExternalSSD/buildbot-root/aarch64-darwin/build/bin/llc -O3
# .---command stderr------------
# | PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace and instructions to reproduce the bug.
# | Stack dump:
# | 0.	Program arguments: /Volumes/ExternalSSD/buildbot-root/aarch64-darwin/build/bin/llc -O3
# | 1.	Running pass 'Function Pass Manager' on module '<stdin>'.
# | 2.	Running pass 'AArch64 Assembly Printer' on function '@test'
# |  #0 0x0000000104425724 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/Volumes/ExternalSSD/buildbot-root/aarch64-darwin/build/bin/llc+0x101725724)
# |  #1 0x00000001044234d4 llvm::sys::RunSignalHandlers() (/Volumes/ExternalSSD/buildbot-root/aarch64-darwin/build/bin/llc+0x1017234d4)
# |  #2 0x0000000104426224 SignalHandler(int, __siginfo*, void*) (/Volumes/ExternalSSD/buildbot-root/aarch64-darwin/build/bin/llc+0x101726224)
# |  #3 0x00000001812b3584 (/usr/lib/system/libsystem_platform.dylib+0x18047b584)
# |  #4 0x0000000103420c94 llvm::DwarfCompileUnit::constructSubprogramScopeDIE(llvm::DISubprogram const*, llvm::Function const&, llvm::LexicalScope*, llvm::MCSymbol*) (/Volumes/ExternalSSD/buildbot-root/aarch64-darwin/build/bin/llc+0x100720c94)
# |  #5 0x0000000103435ea4 llvm::DwarfDebug::endFunctionImpl(llvm::MachineFunction const*) (/Volumes/ExternalSSD/buildbot-root/aarch64-darwin/build/bin/llc+0x100735ea4)
# |  #6 0x00000001034116fc llvm::DebugHandlerBase::endFunction(llvm::MachineFunction const*) (/Volumes/ExternalSSD/buildbot-root/aarch64-darwin/build/bin/llc+0x1007116fc)
# |  #7 0x00000001033f2e14 llvm::AsmPrinter::emitFunctionBody() (/Volumes/ExternalSSD/buildbot-root/aarch64-darwin/build/bin/llc+0x1006f2e14)
# |  #8 0x0000000102e2d8a8 (anonymous namespace)::AArch64AsmPrinter::runOnMachineFunction(llvm::MachineFunction&) (/Volumes/ExternalSSD/buildbot-root/aarch64-darwin/build/bin/llc+0x10012d8a8)
# |  #9 0x00000001036c0260 llvm::MachineFunctionPass::runOnFunction(llvm::Function&) (/Volumes/ExternalSSD/buildbot-root/aarch64-darwin/build/bin/llc+0x1009c0260)
# | #10 0x0000000103bad980 llvm::FPPassManager::runOnFunction(llvm::Function&) (/Volumes/ExternalSSD/buildbot-root/aarch64-darwin/build/bin/llc+0x100ead980)
# | #11 0x0000000103bb4c10 llvm::FPPassManager::runOnModule(llvm::Module&) (/Volumes/ExternalSSD/buildbot-root/aarch64-darwin/build/bin/llc+0x100eb4c10)
# | #12 0x0000000103bae2ac llvm::legacy::PassManagerImpl::run(llvm::Module&) (/Volumes/ExternalSSD/buildbot-root/aarch64-darwin/build/bin/llc+0x100eae2ac)
# | #13 0x0000000102d0767c main (/Volumes/ExternalSSD/buildbot-root/aarch64-darwin/build/bin/llc+0x10000767c)
# | #14 0x0000000180ef7154
# `-----------------------------
# error: command failed with exit status: -11

--

********************


```

</details>

https://github.com/llvm/llvm-project/pull/173500