Fix the placeholder subclass entry skipping [Bug #18489]

class.c

@@ -1054,8 +1054,7 @@ rb_include_module(VALUE klass, VALUE module)
     if (RB_TYPE_P(klass, T_MODULE)) {
         rb_subclass_entry_t *iclass = RCLASS_SUBCLASSES(klass);
         // skip the placeholder subclass entry at the head of the list
-        if (iclass && iclass->next) {
-            RUBY_ASSERT(!iclass->klass);
+        if (iclass && !iclass->klass) {
             iclass = iclass->next;
         }
 

test/ruby/test_module.rb

@@ -475,6 +475,15 @@ class TestModule < Test::Unit::TestCase
     assert_not_include(mod.ancestor_list, BasicObject)
   end
 
+  def test_module_collected_extended_object
+    m1 = labeled_module("m1")
+    m2 = labeled_module("m2")
+    Object.new.extend(m1)
+    GC.start
+    m1.include(m2)
+    assert_equal([m1, m2], m1.ancestors)
+  end
+
   def test_dup
     OtherSetup.call