minor: explicit a precondition on rootlist->level to please Clang analyzer
minor: explicit a precondition on rootlist->level to please Clang analyzer

globroots.c:98:21: warning: Dereference of undefined pointer value
    e->forward[i] = update[i]->forward[i];
                    ^~~~~~~~~~~~~~~~~~~~~

git-svn-id: e7e61699ce1738538ffebb75374fbea0924bc775@16336 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02

yunxing/ocaml,msprotz/ocaml,yunxing/ocaml,gerdstolpmann/ocaml,eliberis/ocaml,yunxing/ocaml,msprotz/ocaml,msprotz/ocaml,eliberis/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml,msprotz/ocaml,yunxing/ocaml,eliberis/ocaml,gerdstolpmann/ocaml,msprotz/ocaml,eliberis/ocaml,gerdstolpmann/ocaml,yunxing/ocaml,eliberis/ocaml