cex: fix miscellaneous leaks

* src/counterexample.c (unifying_counterexample): Always free
stage3result when it exists.
* src/conflicts.c (report_state_counterexamples): free leaked bitset.
* src/state-item.c (prune_disabled_paths): free leaked queue.

src/conflicts.c

@@ -677,6 +677,7 @@ report_state_counterexamples (const state *s)
                     }
                 }
             }
+          bitset_free (conf);
         }
     }
 }