Fix a crash introduced by 3d5e9ab by adding a nullptr check. (#90301)

Author: rniwa

clang/lib/StaticAnalyzer/Checkers/WebKit/UncountedCallArgsChecker.cpp

@@ -53,10 +53,11 @@ class UncountedCallArgsChecker
       bool shouldVisitTemplateInstantiations() const { return true; }
       bool shouldVisitImplicitCode() const { return false; }
 
-      bool TraverseDecl(Decl *D) {
-        if (isa<ClassTemplateDecl>(D) && isRefType(safeGetName(D)))
+      bool TraverseClassTemplateDecl(ClassTemplateDecl *Decl) {
+        if (isRefType(safeGetName(Decl)))
           return true;
-        return RecursiveASTVisitor<LocalVisitor>::TraverseDecl(D);
+        return RecursiveASTVisitor<LocalVisitor>::TraverseClassTemplateDecl(
+            Decl);
       }
 
       bool VisitCallExpr(const CallExpr *CE) {

clang/test/Analysis/Checkers/WebKit/call-args-regression-traverse-decl-crash.cpp

@@ -0,0 +1,7 @@
+// RUN: %clang_analyze_cc1 -analyzer-checker=alpha.webkit.UncountedCallArgsChecker -verify %s
+// expected-no-diagnostics
+
+template <class Class> struct T;
+template <template <class> class Class, class Type>
+struct T<Class<Type>>
+{ };