Indonesia is probing vote-rigging claims after the discovery of thousands of stray election ballots in neighbouring Malaysia, officials said, as the opposition threatens to challenge next week's poll results over separate voter list irregularities.The General Elections Commission (KPU) has sent a team to Malaysia to investigate as many as 20,000 ballots, including many marked in favour of Indonesian President Joko Widodo, that were found in two locations near the capital Kuala Lumpur.&qu