Author of Xscreensaver says:

I think this [attached patch] should make it not crash.

Though, I think that if you go from 2 screens to 1 screen, and that
one screen is higher rez than the first of the two screens was (like
you go from "1024x768 + 1024x768" to "1280x1024") you might see two
overlapping screen savers.  I'm trying not to care about that, since
I think this whole situation is quite dumb.