Thanks for the bug report in <http://bugs.gnu.org/21245>. Good eye, noticing 
those dangling pointers -- these could cause problems in obscure circumstances 
even without custom debuggers.

I installed the attached patch into the emacs-25 branch, and (if I understand 
things correctly) it should address the issues raised in the bug report so I'll 
close the bug report for now; if I'm wrong and this patch doesn't fix the bugs 
we can always reopen it.