Should only check frozen for Array#pop

src/array.c

@@ -451,7 +451,7 @@ mrb_ary_pop(mrb_state *mrb, mrb_value ary)
 {
   struct RArray *a = mrb_ary_ptr(ary);
 
-  ary_modify(mrb, a);
+  ary_modify_check(mrb, a);
   if (a->len == 0) return mrb_nil_value();
   return a->ptr[--a->len];
 }

test/t/array.rb

@@ -237,6 +237,8 @@ assert('Array#pop', '15.2.12.5.21') do
   assert_nil([].pop)
   assert_equal([1,2], a)
   assert_equal(3, b)
+
+  assert_raise(RuntimeError) { [].freeze.pop }
 end
 
 assert('Array#push', '15.2.12.5.22') do