There were probably three or four startups that I can think of that went under during my time at Facebook. I think that we had gotten to the point where we were so used to moving quickly and evolving our technology that the concept of a startup disappearing was really not a show stopper. We knew that we could move beyond them - just disappointed when it happened. I think the bigger risk was when startups would get acquired and that their technology would get into a zombie state. It would be still there, still functional but it wouldn't get the same level of attention and innovation that it was when it was run by the start up.
It also helps if you approach the startup as if you're going to acquire them. Who are the founders / co-founders? What is their track record? How long has the company been in business? What is the overall strategy of the company? Who are their competitors? What is this community saying about the startup, if any? Who are their major customers? etc. By taking such an approach, ideally you'll have a mix of qualitative and quantitate inputs to make a better informed decision. BTW, I've never seen code from the startup being put into escrow actually work. However, it keeps the lawyers happy.