Was that a security issue or just a crash?
Edit: Just to add a bit more detail. xscreensaver locks the screen by putting a window across it that grabs ownership of input devices. Thus if it crashes, the window goes away and the inputs released.
I read about that bug a while ago. It was due to mishandling a strange X11-specific cornercase; to be fair, that's the kind of stuff that is certainly more aptly handled (in the X11 world) in a toolkit (X11-specific cornercases are like 30% of the reason why, as soon as alternatives to Xlib were available, everyone embraced them and cried tears of joy), which would suggest that not relying on toolkits opens xscreensaver's locker to other issues that are more aptly handled in a toolkit.
However, if you look over the bugs that Gnome, Cinnamon or Unity's wrappers had... I'm inclined to think that there are a lot more trivial problems (like the one I linked in my other comment, here : https://news.ycombinator.com/item?id=11412688 ) in Gnome's a thousand and one libraries than there are cornercases in X11.