Coinbase users reported that their account balances showed $0 after bitcoin saw significant gains prior on Wednesday.The exchange said it was investigating the issue shortly after.At least some customers of Coinbase's U.S. cryptocurrency exchange are showing $0 account balances Wednesday as bitcoin (BTC) posted – and then quickly erased – a gigantic rally.The exchange said on its website that it was investigating the matter."We are aware that some users may see a zero balance across their Coinba...