Fixed a bug that resulted when a callback said to halt a search; the level_count was decremented below zero because the found node had not been put in the level history before the callback broke out of the search.
Fixed a bug that resulted when a callback said to halt a search; the
level_count was decremented below zero because the found node had not
been put in the level history before the callback broke out of the search.

Added a bunch of INSISTS()s to try to catch something like that in the future.

pecharmin/bind9,pecharmin/bind9,each/bind9-collab,each/bind9-collab,pecharmin/bind9,each/bind9-collab,each/bind9-collab,pecharmin/bind9,pecharmin/bind9,each/bind9-collab,each/bind9-collab,pecharmin/bind9