hash.c: restore iter_lev

* hash.c (hash_foreach_ensure): restore iter_lev to the previous
  value, not just decrement.  [ruby-dev:47803] [Bug #9105]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43675 b2dd03c8-39d4-4d8f-98ff-823fe69b080e