Don't modify rest array when using ruby2_keywords

Previously, the rest array was modified, but it turns out that is
not necessary.  Not modifying the rest array fixes cases when the
rest array is used more than once.

test/ruby/test_keyword.rb

@@ -2649,6 +2649,11 @@ class TestKeywordArguments < Test::Unit::TestCase
         baz(*args)
       end
 
+      ruby2_keywords def foo_baz2(*args)
+        baz(*args)
+        baz(*args)
+      end
+
       ruby2_keywords def foo_foo_bar(meth, *args)
         foo_bar(meth, *args)
       end
@@ -2761,6 +2766,10 @@ class TestKeywordArguments < Test::Unit::TestCase
     h1 = {a: 1}
     o = c.new
 
+    assert_equal([1, h1], o.foo_baz2(1, :a=>1))
+    assert_equal([1], o.foo_baz2(1, **{}))
+    assert_equal([h1], o.foo_baz2(h1, **{}))
+
     assert_equal([[1], h1], o.foo(:bar, 1, :a=>1))
     assert_equal([1, h1], o.foo(:baz, 1, :a=>1))
     assert_equal([[1], h1], o.bfoo(:bar, 1, :a=>1))

vm_args.c

@@ -742,7 +742,6 @@ setup_parameters_complex(rb_execution_context_t * const ec, const rb_iseq_t * co
             if (RB_TYPE_P(rest_last, T_HASH) &&
                 (((struct RHash *)rest_last)->basic.flags & RHASH_PASS_AS_KEYWORDS)) {
                 rest_last = rb_hash_dup(rest_last);
-                RARRAY_ASET(args->rest, len - 1, rest_last);
                 kw_flag |= VM_CALL_KW_SPLAT;
                 if (iseq->body->param.flags.ruby2_keywords) {
                     remove_empty_keyword_hash = 0;
@@ -754,7 +753,7 @@ setup_parameters_complex(rb_execution_context_t * const ec, const rb_iseq_t * co
         }
 
         if (kw_flag & VM_CALL_KW_SPLAT) {
-            if (len > 0 && ignore_keyword_hash_p(RARRAY_AREF(args->rest, len - 1), iseq)) {
+            if (len > 0 && ignore_keyword_hash_p(rest_last, iseq)) {
                 if (given_argc != min_argc) {
                     if (remove_empty_keyword_hash) {
                         arg_rest_dup(args);