re.c: stop a wrong warning of "flags ignored" on Regexp.new(//)

[Bug #18669]

re.c

@@ -3557,7 +3557,7 @@ rb_reg_initialize_m(int argc, VALUE *argv, VALUE self)
     if (RB_TYPE_P(src, T_REGEXP)) {
 	VALUE re = src;
 
-	if (opts != Qundef) {
+	if (opts != Qnil) {
 	    rb_warn("flags ignored");
 	}
 	rb_reg_check(re);

test/ruby/test_regexp.rb

@@ -557,6 +557,8 @@ class TestRegexp < Test::Unit::TestCase
   def test_initialize
     assert_raise(ArgumentError) { Regexp.new }
     assert_equal(/foo/, assert_warning(/ignored/) {Regexp.new(/foo/, Regexp::IGNORECASE)})
+    assert_equal(/foo/, assert_no_warning(/ignored/) {Regexp.new(/foo/)})
+    assert_equal(/foo/, assert_no_warning(/ignored/) {Regexp.new(/foo/, timeout: nil)})
 
     assert_equal(Encoding.find("US-ASCII"), Regexp.new("b..", nil, "n").encoding)
     assert_equal("bar", "foobarbaz"[Regexp.new("b..", nil, "n")])