Check the encoding of `ruby2_keywords_flag` [Bug #18184]

marshal.c

@@ -1422,6 +1422,7 @@ ruby2_keywords_flag_check(VALUE sym)
 {
     const char *p;
     long l;
+    if (rb_enc_get_index(sym) != ENCINDEX_US_ASCII) return 0;
     RSTRING_GETMEM(sym, p, l);
     if (l <= 0) return 0;
     if (name_equal(name_s_ruby2_keywords_flag, rb_strlen_lit(name_s_ruby2_keywords_flag), p, l)) {

test/ruby/test_marshal.rb

@@ -810,7 +810,11 @@ class TestMarshal < Test::Unit::TestCase
     hash = Marshal.load(data)
     assert_equal(42, ruby2_keywords_test(*[hash]))
 
-    hash2 = Marshal.load(data.sub!(/\x06K(?=T\z)/, "\x08KEY"))
+    hash2 = Marshal.load(data.sub(/\x06K(?=T\z)/, "\x08KEY"))
+    assert_raise(ArgumentError, /\(given 1, expected 0\)/) {
+      ruby2_keywords_test(*[hash2])
+    }
+    hash2 = Marshal.load(data.sub(/:\x06K(?=T\z)/, "I\\&\x06:\x0dencoding\"\x0dUTF-16LE"))
     assert_raise(ArgumentError, /\(given 1, expected 0\)/) {
       ruby2_keywords_test(*[hash2])
     }