clear `kw_flag` if given hash is nil

https://bugs.ruby-lang.org/issues/20570 is caused I missed to
clear the `kw_flag` even if `keyword_hash` is nil.

test/ruby/test_keyword.rb

@@ -4523,6 +4523,24 @@ class TestKeywordArgumentsSymProcRefinements < Test::Unit::TestCase
     assert_equal({one: 1, two: 2}, f.call(one:, two:))
   end
 
+  def m_bug20570(*a, **nil)
+    a
+  end
+
+  def test_splat_arg_with_prohibited_keyword
+    assert_equal([], m_bug20570(*[]))
+    assert_equal([1], m_bug20570(*[1]))
+    assert_equal([1, 2], m_bug20570(*[1, 2]))
+    h = nil
+    assert_equal([], m_bug20570(*[], **h))
+    assert_equal([1], m_bug20570(*[1], **h))
+    assert_equal([1, 2], m_bug20570(*[1, 2], **h))
+
+    assert_equal([], m_bug20570(*[], **nil))
+    assert_equal([1], m_bug20570(*[1], **nil))
+    assert_equal([1, 2], m_bug20570(*[1, 2], **nil))
+  end
+
   private def one
     1
   end

vm_args.c

@@ -524,10 +524,9 @@ static inline int
 ignore_keyword_hash_p(VALUE keyword_hash, const rb_iseq_t * const iseq, unsigned int * kw_flag, VALUE * converted_keyword_hash)
 {
     if (keyword_hash == Qnil) {
-        return 1;
+        goto ignore;
     }
-
-    if (!RB_TYPE_P(keyword_hash, T_HASH)) {
+    else if (!RB_TYPE_P(keyword_hash, T_HASH)) {
         keyword_hash = rb_to_hash_type(keyword_hash);
     }
     else if (UNLIKELY(ISEQ_BODY(iseq)->param.flags.anon_kwrest)) {
@@ -543,9 +542,17 @@ ignore_keyword_hash_p(VALUE keyword_hash, const rb_iseq_t * const iseq, unsigned
         keyword_hash = rb_hash_dup(keyword_hash);
     }
     *converted_keyword_hash = keyword_hash;
-    return !(ISEQ_BODY(iseq)->param.flags.has_kw) &&
+
+    if (!(ISEQ_BODY(iseq)->param.flags.has_kw) &&
         !(ISEQ_BODY(iseq)->param.flags.has_kwrest) &&
-           RHASH_EMPTY_P(keyword_hash);
+        RHASH_EMPTY_P(keyword_hash)) {
+      ignore:
+        *kw_flag &= ~(VM_CALL_KW_SPLAT | VM_CALL_KW_SPLAT_MUT);
+        return 1;
+    }
+    else {
+        return 0;
+    }
 }
 
 static VALUE
@@ -699,7 +706,6 @@ setup_parameters_complex(rb_execution_context_t * const ec, const rb_iseq_t * co
                         rb_ary_pop(args->rest);
                     }
                     given_argc--;
-                    kw_flag &= ~(VM_CALL_KW_SPLAT | VM_CALL_KW_SPLAT_MUT);
                 }
                 else {
                     if (rest_last != converted_keyword_hash) {
@@ -730,7 +736,6 @@ setup_parameters_complex(rb_execution_context_t * const ec, const rb_iseq_t * co
             if (ignore_keyword_hash_p(last_arg, iseq, &kw_flag, &converted_keyword_hash)) {
                 args->argc--;
                 given_argc--;
-                kw_flag &= ~(VM_CALL_KW_SPLAT | VM_CALL_KW_SPLAT_MUT);
             }
             else {
                 if (!(kw_flag & VM_CALL_KW_SPLAT_MUT) && !ISEQ_BODY(iseq)->param.flags.has_kw) {