catalog zones reconfig crash

With a configuration like https://www.dns.cam.ac.uk/code/nsconfig/catz.named.conf I can crash named by disabling and re-enabling the catalog zone.

I comment out the catalog-zones clause to disable it, and run rndc reconfig; then I uncomment the catalog-zones clause to re-enable it, then when I run rndc reconfig again, named crashes.

2020-02-07.15:43:47.596 general: critical: zone.c:1806: INSIST(zone->catzs == ((void *)0) || zone->catzs == catzs) failed, back trace

I am running BIND 9.15.8 revision 9943c5dc plus patches

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