Fix crash when using embedded DWARF-5 debugging info

[amadio]

Summary

A nullptr dereference happens in DIFile when Source is non-null, but
*Source is null, as only Source is checked. The crash happens when
using embedded DWARF-5 debugging info in combination with the GDB
JIT event listener to debug JIT-compiled code within cling/ROOT.

[amadio]

Updated due to clang-format failure.

[v.g.vassilev]

This LGTM but I'd like to add reviewers who changed this area recently.

[dblaikie]

Any chance of a test case? (unit test using the JIT event listener directly, perhaps?)

[Hahnfeld]

As far as I could tell with some testing, this crash happens if you pass in an empty string because then getCanonicalMDString() returns a nullptr. I think it would be cleaner to normalize this case to None: https://reviews.llvm.org/D138658

[amadio]

Thanks for looking into this. I'm going to close this in favor of D138658. We should revert my change in ROOT and apply your patch instead.

llvm/include/llvm/IR/DebugInfoMetadata.h

     return StringRefChecksum;
   }
   Optional<StringRef> getSource() const {
-    return Source ? Optional<StringRef>((*Source)->getString()) : None;
+    return (Source && *Source) ? Optional<StringRef>((*Source)->getString()) : None;
   }
 
   MDString *getRawFilename() const { return getOperandAs<MDString>(0); }