Fix class variable reference in module

Fix #3079

src/variable.c

@@ -773,7 +773,7 @@ mrb_mod_cv_get(mrb_state *mrb, struct RClass * c, mrb_sym sym)
     klass = mrb_obj_iv_get(mrb, (struct RObject *)cls,
                            mrb_intern_lit(mrb, "__attached__"));
     c = mrb_class_ptr(klass);
-    if (c->tt == MRB_TT_CLASS) {
+    if (c->tt == MRB_TT_CLASS || c->tt == MRB_TT_MODULE) {
       while (c) {
         if (c->iv && iv_get(mrb, c->iv, sym, &v)) {
           return v;

test/t/class.rb

@@ -384,6 +384,19 @@ assert('class variable and class << self style class method') do
   assert_equal("value", ClassVariableTest.class_variable)
 end
 
+assert('class variable in module and class << self style class method') do
+  module ClassVariableInModuleTest
+    @@class_variable = "value"
+    class << self
+      def class_variable
+        @@class_variable
+      end
+    end
+  end
+
+  assert_equal("value", ClassVariableInModuleTest.class_variable)
+end
+
 assert('class with non-class/module outer raises TypeError') do
   assert_raise(TypeError) { class 0::C1; end }
   assert_raise(TypeError) { class []::C2; end }