[Clang][Sema] Add a nullptr check in template name (llvm#162377)

Static analysis flagged that when calling ActOnTemplateName, `S` can be
a `nullptr`
and we call `isTemplateName` which unconditionally dereferences the `S`
argument at
some point. I added a `nullptr` check to assure we don't dereference `S`
in
`isTemplateName` if it is a `nullptr`.

Author: shafik

clang/lib/Sema/SemaTemplate.cpp

@@ -318,7 +318,7 @@ TemplateNameKind Sema::isTemplateName(Scope *S,
     }
   }
 
-  if (isPackProducingBuiltinTemplateName(Template) &&
+  if (isPackProducingBuiltinTemplateName(Template) && S &&
       S->getTemplateParamParent() == nullptr)
     Diag(Name.getBeginLoc(), diag::err_builtin_pack_outside_template) << TName;
   // Recover by returning the template, even though we would never be able to