bound checking bug with caml_string_{get,set}{16,32,64}: fix the runtime C code

As notified by Nicolas Trangez, the following program behaves in an
unexpected way by returning 0 instead of failing with an out-of-bound
exception:

  external get16 : string -> int -> int = "%caml_string_get16"
  let test = get16 "" 0

caml_string_get16(str, idx) will access indices (idx) and (idx+1). The
bound-checking code is currently implemented as:

   if (idx < 0 || idx >= caml_string_length(str) - 1) caml_array_bound_error();

This is wrong as caml_string_length returns an mlsize_t which is
unsigned, so substracting 1 gets buggy when the size is 0. The test
should be written as follow:

   if (idx < 0 || idx + 1 >= caml_string_length(str)) caml_array_bound_error();

Note 1: we can exploit this bug to make out-of-bound access to
a string, but I think get16 examples will run into the padding
characters of OCaml strings and behave in a not-too-wrong way. It may
also be the case of get32, but get64 will access 7 bytes, so access
memory outside the string:

  # external set64: string -> int -> int -> unit = "%caml_string_get64";;
  external set64 : string -> int -> int -> unit = "%caml_string_get64"
  # set64 "" 0 0;;
  Segmentation fault

Note 2: this first commit only fixes the C code in byterun/str.c. Only
ocamlc actually uses these functions when the compiler primitive is
used ("%caml_string_get16" instead of "caml_string_get16"). ocamlopt
generates ocaml code directly, and this part has yet to be fixed in
a following commit.

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14267 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02