I'm not saying it isn't hard - I acknowledge that getting it right is a huge task. But the cost of failure should be rendering incorrectly, not crashing or memory corruption.
There was a similar bug about a year ago as well. I'm surprised more fuzzing, static analysis, and general testing hasn't gone into this area since there have been issues found there already. Maybe they have done this extra work and just didn't catch this one. It's hard to know. Still feels like the way it fails is unacceptable.