I don't believe the X system had/has a separate protocol for screen locking, or if it does, that any of the programs implement it. So xscreensaver is just another X client that happens to draw itself full-screen on top of all other apps and grab all user input.
From the point of view of the display manager, a screensaver/screenlocker crashing is just a simple app crash. There's nothing in the protocol to suggest that this is a security failure.
From the point of view of the display manager, a screensaver/screenlocker crashing is just a simple app crash. There's nothing in the protocol to suggest that this is a security failure.