Guard against nullptr (#94084)

Protect against nullptr after #93926

Author: Sterling-Augustine

clang/lib/AST/QualTypeNames.cpp

@@ -65,8 +65,9 @@ static bool getFullyQualifiedTemplateName(const ASTContext &Ctx,
   assert(ArgTDecl != nullptr);
   QualifiedTemplateName *QTName = TName.getAsQualifiedTemplateName();
 
-  if (QTName && !QTName->hasTemplateKeyword()) {
-    NNS = QTName->getQualifier();
+  if (QTName &&
+      !QTName->hasTemplateKeyword() &&
+      (NNS = QTName->getQualifier())) {
     NestedNameSpecifier *QNNS = getFullyQualifiedNestedNameSpecifier(
         Ctx, NNS, WithGlobalNsPrefix);
     if (QNNS != NNS) {