Set a new bytes used for elliptic curves computations to '\000'

Cstruct.create does this. If we don't initialize bytes with '\000',
Field_element.zero can be something else than '\000'. It's a fix for
mirleft/ocaml-x509#167.

ec/mirage_crypto_ec.ml

@@ -135,7 +135,7 @@ end
 module Make_field_element (P : Parameters) (F : Foreign) : Field_element = struct
   let b_uts b = Bytes.unsafe_to_string b
 
-  let create () = Bytes.create P.fe_length
+  let create () = Bytes.make P.fe_length '\000'
 
   let mul a b =
     let tmp = create () in