By Rodrigo Campos NEW YORK (Reuters) – A positive vibe returned to the U.S. stock market Friday, leaving some to wonder if, after two weeks of losses, the latest selloff scare was over. The best clues may come from what happens to low-quality corporate bonds. The most recent decline in the S&P 500 marks the third time in six months that the market has looked wobbly and threatened a significant reversal. Each time, so far, it has bounced back quickly. But what has some investors most worried this time around is the recent, notable underperformance in junk bonds in the past few months. ..