Check a list length before doing a List.chop (fix #3000)

 I'm not completely sure that raising Not_found is the right
 thing to do here, but it seems reasonable...

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/branches/v8.4@16327 85f007b7-540e-0410-9357-904b9bb8a0f7