Fix fork crash with no init, test for (#4471)

src/V3Fork.cpp

@@ -218,10 +218,12 @@ private:
         beginp->stmtsp()->addNext(instAsgnp);
         beginp->stmtsp()->addNext(forkp);
 
-        forkp->initsp()->foreach([forkp](AstAssign* asgnp) {
-            asgnp->unlinkFrBack();
-            forkp->addHereThisAsNext(asgnp);
-        });
+        if (forkp->initsp()) {
+            forkp->initsp()->foreach([forkp](AstAssign* asgnp) {
+                asgnp->unlinkFrBack();
+                forkp->addHereThisAsNext(asgnp);
+            });
+        }
         UASSERT_OBJ(!forkp->initsp(), forkp, "Leftover nodes in block_item_declaration");
 
         m_modp->addStmtsp(m_instance.m_classp);

test_regress/t/t_fork_initial.pl

@@ -0,0 +1,26 @@
+#!/usr/bin/env perl
+if (!$::Driver) { use FindBin; exec("$FindBin::Bin/bootstrap.pl", @ARGV, $0); die; }
+# DESCRIPTION: Verilator: Verilog Test driver/expect definition
+#
+# Copyright 2023 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
+
+scenarios(simulator => 1);
+
+compile(
+    verilator_flags2 => ["--exe --main --timing"],
+    make_main => 0,
+    # bug#4471 - remove this
+    verilator_make_gmake => 0,
+    );
+
+#bug#4471 - add this
+#execute(
+#    check_finished => 1,
+#    );
+
+ok(1);
+1;

test_regress/t/t_fork_initial.v

@@ -0,0 +1,15 @@
+// DESCRIPTION: Verilator: Verilog Test module
+//
+// This file ONLY is placed under the Creative Commons Public Domain, for
+// any use, without warranty, 2023 by Wilson Snyder.
+// SPDX-License-Identifier: CC0-1.0
+
+module t();
+  initial fork
+     reg i;
+     i = 1'b1;
+     if (i != 1'b1) $stop;
+     $write("*-* All Finished *-*\n");
+     $finish;
+  join
+endmodule