[verification] Any invalid sub-formula results into invalid formula.

src/verification.ml

@@ -178,6 +178,7 @@ let combine_prover_answers answers =
   List.fold_left answers ~init:Call_provers.Valid ~f:(fun acc r ->
     match (acc, r) with
     | Call_provers.Valid, r | r, Call_provers.Valid -> r
+    | Invalid, _ | _, Invalid -> Invalid
     | _ -> acc)
 
 let answer_dataset limit config env prover config_prover driver dataset task =