Fix FrozenError when assigning frozen class to constant

* variable.c (set_namespace_path): modules/classes can get named
  by assignment to constant, even if frozen.  [Bug #15891]

test/ruby/test_module.rb

@@ -584,6 +584,8 @@ class TestModule < Test::Unit::TestCase
     assert_equal(prefix+"N", m.const_get(:N).name)
     assert_equal(prefix+"O", m.const_get(:O).name)
     assert_equal(prefix+"C", m.const_get(:C).name)
+    c = m.class_eval("Bug15891 = Class.new.freeze")
+    assert_equal(prefix+"Bug15891", c.name)
   end
 
   def test_private_class_method

variable.c

@@ -2733,8 +2733,10 @@ static void
 set_namespace_path(VALUE named_namespace, VALUE namespace_path)
 {
     struct rb_id_table *const_table = RCLASS_CONST_TBL(named_namespace);
-
-    rb_ivar_set(named_namespace, classpath, namespace_path);
+    if (!RCLASS_IV_TBL(named_namespace)) {
+        RCLASS_IV_TBL(named_namespace) = st_init_numtable();
+    }
+    rb_class_ivar_set(named_namespace, classpath, namespace_path);
     if (const_table) {
         rb_id_table_foreach(const_table, set_namespace_path_i, &namespace_path);
     }