* eval.c (setup_exception): "mesg == sysstack_error" and
  sysstack_error_p(mesg) are duplicated.
  r46502 seems to want to use latter.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46531 b2dd03c8-39d4-4d8f-98ff-823fe69b080e