class.c: trivial optimization

* class.c (singleton_class_of): just copy FROZEN flag without
  conditions.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@56739 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

class.c

@@ -1619,7 +1619,7 @@ singleton_class_of(VALUE obj)
     else {
 	FL_UNSET(klass, FL_TAINT);
     }
-    if (OBJ_FROZEN(obj)) OBJ_FREEZE_RAW(klass);
+    RB_FL_SET_RAW(klass, RB_OBJ_FROZEN_RAW(obj));
 
     return klass;
 }