Do the right thing on NULL uint64 fields.

Patch by Clemens Hammacher!

Fixes PR12243

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@152880 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/test/DebugInfo/bug_null_debuginfo.ll b/test/DebugInfo/bug_null_debuginfo.ll
new file mode 100644
index 0000000..a7fdf70
--- /dev/null
+++ b/test/DebugInfo/bug_null_debuginfo.ll
@@ -0,0 +1,6 @@
+; RUN: llc
+
+
+!llvm.dbg.cu = !{!0}
+
+!0 = metadata !{null, null, null}