handler: fix memory leak and use-after-free errors

The errors are related to newly added handler_system_delete_all().

src/handler.c

@@ -686,4 +686,5 @@ void handler_system_delete_all(xmpp_conn_t *conn)
             xmpp_free(conn->ctx, key2);
         }
     }
+    if (iter) hash_iter_release(iter);
 }