Catch stack pointers that fall clearly outside of the kernel address space and abort kernel stack unwinding rather than continuing. For failed stack unwinding in the kernel debugger, keeling over just causes the debugger to re-enter. But for production uses of stack unwinding such as PMC and DTrace, a bad pointer dereference is less acceptable behaviour.
Hmm. Good question. I think that aspect of the change originated with you or br, but presumably reflects an actual case that arose. I can remove it, but perhaps br will remember and be able to comment?