Also in ClearNonLiveTransitions we have to check if there still is a transition array before we access it.
This should never happen in the long run, but will happen when maps still have a back-pointer while the forward pointer (the transition) was overwritten.

Review URL: https://chromiumcodereview.appspot.com/10704109

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@12000 ce2b1a6d-e550-0410-aec6-3dcde31c8c00