Fix caml_string_{get,set}{16,32,64} bound checking in cmmgen
    
    (Patch by Pierre Chambart)
    
    Followup for trunk@14267:
      bound checking bug with caml_string_{get,set}{16,32,64}: fix the runtime C code
    
    git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14556 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02