Add proper casts.

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14304 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
Fix PR#6241

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14305 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02