fix: check scan return

src/thread.c

@@ -20,7 +20,8 @@ void *routine(void *p_data)
 		scan_data.port = port;
 		scan_data.response =
 		    &thread_data->responses[thread_data->port_start - port];
-		scan(&scan_data);
+		if (scan(&scan_data))
+			return NULL;
 		printf("state of port %d: %d\n", port,
 		       scan_data.response->state);
 	}