vm_trace.c: isolate exceptions

* vm_trace.c (rb_postponed_job_flush): isolate exceptions in
  postponed jobs and restore outer ones.  based on a patch by
  tarui.  [ruby-core:58652] [Bug #9168]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44260 b2dd03c8-39d4-4d8f-98ff-823fe69b080e