fix findall/4 (#873)

src/lib/builtins.pl

@@ -682,9 +682,8 @@ findall(Template, Goal, Solutions) :-
     ).
 
 
-% :- meta_predicate '$iterate_find_all_diff'(?, 0, ?, ?, ?).
-
 :- non_counted_backtracking '$iterate_find_all_diff'/5.
+
 '$iterate_find_all_diff'(Template, Goal, _, _, LhOffset) :-
     call(Goal),
     '$copy_to_lh'(LhOffset, Template),

src/machine/system_calls.rs

@@ -3267,8 +3267,7 @@ impl MachineState {
                             let solutions = self[temp_v!(2)];
                             let diff = self[temp_v!(3)];
 
-                            (self.unify_fn)(self, solutions, Addr::EmptyList);
-                            (self.unify_fn)(self, diff, Addr::EmptyList);
+                            (self.unify_fn)(self, solutions, diff);
                         } else {
                             let h = self.heap.h();
                             let mut last_index = h;