* vm.c (vm_exec): get rid of a SEGV when calling rb_iter_break() from
  some extention libraries.  [Backport #7896] [ruby-core:52607]


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_1_9_3@39381 b2dd03c8-39d4-4d8f-98ff-823fe69b080e