hash.c: iteration level with reentering

* hash.c (hash_foreach_iter, hash_foreach_ensure, rb_hash_foreach):
  deal with iteration level when reentering by callcc.  temporary
  measure until rollback of ensure is introduced.  [ruby-dev:47807]
  [Bug #9105]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43683 b2dd03c8-39d4-4d8f-98ff-823fe69b080e