Out-of-bounds error for top-level splices in module bodies

[yallop]

This is with the macros_old branch.

For example,

$ cat s.ml
module M = struct
  let a = $(Expr.of_int 0)
end
$ ocaml s.ml 
Fatal error: exception Invalid_argument("index out of bounds")

[yallop]

The same bug is still present on the macros_with_lambda_quotes branch.