[vnnlib] Just simplify already negated formula.

src/printers/vnnlib.ml

@@ -435,6 +435,7 @@ let print_goal info fmt (pr, t) =
 
 let rec negate_term info t =
   match t.Term.t_node with
+  | Tnot t -> t
   | Tbinop (Tand, t1, t2) ->
     Term.t_or (negate_term info t1) (negate_term info t2)
   | Tbinop (Tor, t1, t2) ->