Fix crash when super.new() called without a base class (#6772)

src/V3LinkDot.cpp

@@ -4851,7 +4851,7 @@ class LinkDotResolveVisitor final : public VNVisitor {
             iterateChildren(nodep);
             if (isNew) {
                 const AstClassExtends* const classExtendsp = VN_AS(m_modp, Class)->extendsp();
-                if (m_explicitSuperNewp && !m_explicitSuperNewp->isImplicit()
+                if (m_explicitSuperNewp && !m_explicitSuperNewp->isImplicit() && classExtendsp
                     && classExtendsp->argsp()) {
                     m_explicitSuperNewp->v3error(
                         "Explicit super.new not allowed with class "

test_regress/t/t_class_super_new_noextend_bad.out

@@ -0,0 +1,5 @@
+%Error: t/t_class_super_new_noextend_bad.v:10:12: 'super' used on non-extended class (IEEE 1800-2023 8.15)
+   10 |       super.new();   
+      |            ^
+        ... See the manual at https://verilator.org/verilator_doc.html?v=latest for more assistance.
+%Error: Exiting due to

test_regress/t/t_class_super_new_noextend_bad.py

@@ -0,0 +1,16 @@
+#!/usr/bin/env python3
+# DESCRIPTION: Verilator: Verilog Test driver/expect definition
+#
+# Copyright 2025 by Wilson Snyder. This program is free software; you
+# can redistribute it and/or modify it under the terms of either the GNU
+# Lesser General Public License Version 3 or the Perl Artistic License
+# Version 2.0.
+# SPDX-License-Identifier: LGPL-3.0-only OR Artistic-2.0
+
+import vltest_bootstrap
+
+test.scenarios('linter')
+
+test.lint(fails=True, expect_filename=test.golden_filename)
+
+test.passes()

test_regress/t/t_class_super_new_noextend_bad.v

@@ -0,0 +1,12 @@
+// DESCRIPTION: Verilator: Verilog Test module
+//
+// This file ONLY is placed under the Creative Commons Public Domain, for
+// any use, without warranty, 2025 by Wilson Snyder.
+// SPDX-License-Identifier: CC0-1.0
+//
+
+class Cls;
+   function new();
+      super.new();  // Bad - no extends
+   endfunction
+endclass