Fix attr_reader-generated methods accepting extra arguments

attr_reader-generated getter methods silently ignored any arguments
passed to them. CRuby raises ArgumentError in this case.

Add mrb_get_args(mrb, "") to enforce zero arguments, matching CRuby.

src/class.c

@@ -2776,6 +2776,7 @@ mod_attr_define(mrb_state *mrb, mrb_value mod, mrb_value (*accessor)(mrb_state*,
 static mrb_value
 attr_reader(mrb_state *mrb, mrb_value obj)
 {
+  mrb_get_args(mrb, "");
   mrb_value name = mrb_proc_cfunc_env_get(mrb, 0);
   return mrb_iv_get(mrb, obj, to_sym(mrb, name));
 }

test/t/module.rb

@@ -163,6 +163,9 @@ assert('Module#attr_reader', '15.2.2.4.13') do
 
   AttrTestReader.cattr_val = 'test'
   assert_equal 'test', AttrTestReader.cattr
+
+  assert_raise(ArgumentError) { attr_instance.iattr(1) }
+  assert_raise(ArgumentError) { attr_instance.iattr(1, 2, 3) }
 end
 
 assert('Module#attr_writer', '15.2.2.4.14') do