fix automatic length calculation when offset is provided

this bug was exposed by the added tests.

Author: Christopher Zimmermann

src/bigstring_unix.ml

@@ -9,7 +9,7 @@ let get_bounds name ?(off=0) ?len t =
  let buffer_len = Bigarray.Array1.dim t in
  let len = match len with
  | Some len -> len
- | None -> buffer_len
+ | None -> buffer_len - off
  in
  if len < 0 || off < 0 || buffer_len - off < len
  then invalid_arg ("Bigstring_unix." ^ name)