eval.c: no overwrite SystemStackError backtrace

* eval.c (setup_exception): should not overwrite SystemStackError
  backtrace if set already.  [ruby-core:63377] [Feature #6216]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46594 b2dd03c8-39d4-4d8f-98ff-823fe69b080e