[clang] Fix a crash introduced by 3d5e9ab by adding a nullptr check. (PR #90301)

[Matheus Izvekov via cfe-commits]

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@@ -54,7 +54,7 @@ class UncountedCallArgsChecker
       bool shouldVisitImplicitCode() const { return false; }
 
       bool TraverseDecl(Decl *D) {
-        if (isa<ClassTemplateDecl>(D) && isRefType(safeGetName(D)))
+        if (D && isa<ClassTemplateDecl>(D) && isRefType(safeGetName(D)))
           return true;
         return RecursiveASTVisitor<LocalVisitor>::TraverseDecl(D);
       }
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mizvekov wrote:

Shouldn't this be overriding `TraverseTemplateInstantiations(ClassTemplateDecl *D)` instead?
    
 `RecursiveASTVisitor<LocalVisitor>::TraverseDecl`  will ensure you won't be visiting null nodes.

https://github.com/llvm/llvm-project/pull/90301