eval.c: preserve errinfo

* eval.c (rb_ensure): preserve errinfo accross ensure proc before
  JUMP_TAG().  [ruby-core:52022] [Bug #7802]


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39156 b2dd03c8-39d4-4d8f-98ff-823fe69b080e