Some internet sleuthing led me to this article from 2010 wherein the author has listed cases where being struck by a golf ball led to a fatality. The author's research has provided 8 instances of death by a golf ball over a roughly 17 year period. What is important to note is that all these deaths seem to involve amateur golfers or the unfortunate victims being on the fairway or greens at the wrong time.
On the PGA Tour, I believe this is not a major hazard as the chances of wayward shots into (significantly large) crowds would be negligible considering the skill level of the players. My research has not uncovered incidents of golf ball induced death on the PGA Tour, so it seems that this is a negligible hazard for crowds at PGA Tour events.
Some more research led me to this feelgood story from May 2016 where an onlooker was hit on the head by PGA Tour golfer Tony Finau. There are a few more instances of such injuries on the PGA Tour. However, the odds of them being fatal are significantly less as such events usually have a competent medical staff available during the tournament, which greatly reduces the chances of injuries to players or onlookers turning fatal.