Add a frozenness check to Enumerator#initialize.

* enumerator.c (enumerator_init): Add a frozenness check to
  prevent a frozen Enumerator object from being reinitialized with
  a different enumerable object.  This is the least we should do,
  and more fixes will follow. [Fixes GH-368] Patch by Kenichi
  Kamiya.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42233 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
Add a frozenness check to Enumerator::Generator#initialize.

* enumerator.c (generator_init): Ditto.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42234 b2dd03c8-39d4-4d8f-98ff-823fe69b080e