vm_eval.c: suppress warning

* vm_eval.c (eval_string_with_cref): move cref inside EXEC_TAG block
  to suppress a warning that 'cref' might be clobbered by 'longjmp',
  on gcc 4.7 or later.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42582 b2dd03c8-39d4-4d8f-98ff-823fe69b080e