FRANKFURT (Reuters) - Euro zone regulators should consider forcing banks to build bigger capital buffers as protection against an even bigger downturn that could lead to a credit crunch, European Central Bank Vice President Luis de Guindos said on Wednesday. Having amassed capital over the last decade, the currency bloc's biggest banks are now well positioned to weather a downturn but most still lack an extra capital buffer that could be freed up during periods of stress."Even if we consider the..