Attaching a marginally cleaner version of the patch, with comments.

I think it's clear this addresses _some_ real fatal problems, but they're rare:  for a problem to show up, it has to be the case that finalize() reduces the refcount of the current object to 0, and that this also causes "the immediately previous" object in the gc list to become free'd, or at least untracked from gc.  Then the next "current object" will be taken from an insane gc_next member, and all bets are off.