don't dereference a null pointer

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@59533 91177308-0d34-0410-b5e6-96231b3b80d8

include/clang/Basic/Diagnostic.h

@@ -214,6 +214,7 @@ class DiagnosticInfo {
 public:
   DiagnosticInfo(Diagnostic *diagObj, FullSourceLoc loc, unsigned diagID) :
     DiagObj(diagObj), Loc(loc), DiagID(diagID) {
+    if (DiagObj == 0) return;
     assert(DiagObj->NumDiagArgs == -1 &&
            "Multiple diagnostics in flight at once!");
     DiagObj->NumDiagArgs = DiagObj->NumDiagRanges = 0;