Fix lazy enumerator with index size

Fixes [Bug #17889]

enumerator.c

@@ -2700,8 +2700,13 @@ lazy_with_index_proc(VALUE proc_entry, struct MEMO* result, VALUE memos, long me
     return result;
 }
 
+static VALUE
+lazy_with_index_size(VALUE proc, VALUE receiver) {
+    return receiver;
+}
+
 static const lazyenum_funcs lazy_with_index_funcs = {
-    lazy_with_index_proc, 0,
+    lazy_with_index_proc, lazy_with_index_size,
 };
 
 /*

test/ruby/test_lazy_enumerator.rb

@@ -682,4 +682,8 @@ EOS
     ary = (0..Float::INFINITY).lazy.with_index.take(2).to_a
     assert_equal([[0, 0], [1, 1]], ary)
   end
+
+  def test_with_index_size
+    assert_equal(3, Enumerator::Lazy.new([1, 2, 3], 3){|y, v| y << v}.with_index.size)
+  end
 end