Do not apply anon_rest optimization when passed array uses keyword-flagged hash

The optimization sets args->rest_dupped to avoid allocating an array,
but this is not safe if the splat array ends in a keyword flagged
hash.  Unset args->rest_dupped in this case.

Fixes [Bug #20388]

test/ruby/test_keyword.rb

@@ -2835,6 +2835,19 @@ class TestKeywordArguments < Test::Unit::TestCase
     assert_equal({a: 1}, kw)
   end
 
+  def test_anon_splat_ruby2_keywords_bug_20388
+    extend(Module.new{def process(action, ...) 1 end})
+    extend(Module.new do
+      def process(action, *args)
+        args.freeze
+        super
+      end
+      ruby2_keywords :process
+    end)
+
+    assert_equal(1, process(:foo, bar: :baz))
+  end
+
   def test_top_ruby2_keywords
     assert_in_out_err([], <<-INPUT, ["[1, 2, 3]", "{:k=>1}"], [])
       def bar(*a, **kw)

vm_args.c

@@ -687,6 +687,9 @@ setup_parameters_complex(rb_execution_context_t * const ec, const rb_iseq_t * co
                 rest_last = rb_hash_dup(rest_last);
                 kw_flag |= VM_CALL_KW_SPLAT | VM_CALL_KW_SPLAT_MUT;
 
+                // Unset rest_dupped set by anon_rest as we may need to modify splat in this case
+                args->rest_dupped = false;
+
                 if (ignore_keyword_hash_p(rest_last, iseq, &kw_flag, &converted_keyword_hash)) {
                     arg_rest_dup(args);
                     rb_ary_pop(args->rest);