fix dangerous code

src/pari.ml

@@ -77,7 +77,7 @@ module Integer = struct
 
   let shift x s = mpshift x (Signed.Long.of_int s)
   let sqrt i = inj_complex (sqrti i)
-  let zero = of_int 0
+  let zero () = of_int 0
 
   let mul x y =
     let res = mulii x y in
@@ -340,7 +340,7 @@ module Integer_mod = struct
    fun x ->
     let x = Ctypes.(coerce gen (ptr gen) x) in
     let modulo = Ctypes.(!@(x +@ 1)) in
-    let res = Ctypes.allocate t Integer.zero in
+    let res = Ctypes.allocate t (Integer.zero ()) in
     if invmod Ctypes.(!@(x +@ 2)) modulo res = 1 then
       Some (create Ctypes.(!@res) ~modulo)
     else None

src/pari.mli

@@ -169,7 +169,7 @@ and Integer : sig
   val equal : t -> t -> bool
   val shift : t -> int -> t
   val sqrt : t -> Real.t
-  val zero : t
+  val zero : unit -> t
   val mul : t -> t -> t
   val add : t -> t -> t
   val sub : t -> t -> t