named crashes in task.c:1110: INSIST(VALID_TASK(task));

A stress-tested BIND 9.15.8 recursive resolver crashed after a few hours with the following logs:

17-Jan-2020 20:34:25.220 task.c:1110: INSIST((__builtin_expect(!!((task) != ((void *)0)), 1) && __builtin_expect(!!(((const isc__magic_t *)(task))->magic == ((('T') << 24 | ('A') << 16 | ('S') << 8 | ('K')))), 1))) failed, back trace
17-Jan-2020 20:34:25.220 #0 0x43636c in assertion_failed()+0x4c
17-Jan-2020 20:34:25.220 #1 0x6224ea in isc_assertion_failed()+0xa
17-Jan-2020 20:34:25.220 #2 0x6475d5 in run()+0xae5
17-Jan-2020 20:34:25.220 #3 0x7fd07c9634e2 in __do_global_dtors_aux_fini_array_entry()+0x7fd07c24abca
17-Jan-2020 20:34:25.220 #4 0x7fd07c88b693 in __do_global_dtors_aux_fini_array_entry()+0x7fd07c172d7b
17-Jan-2020 20:34:25.220 exiting (due to assertion failure)