handler by using ruby_engine_name.  [ruby-core:29497]

* vm_eval.c (print_backtrace): file may be nil when segfaulted in
  very early stage.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@27345 b2dd03c8-39d4-4d8f-98ff-823fe69b080e