Crash in resolver code

Reproduced on 9.14.0 on FreeBSD, real world resolver with Flamethrower generating load:

#0  0x000000080239847a in thr_kill () from /lib/libc.so.7
#1  0x0000000802398444 in raise () from /lib/libc.so.7
#2  0x00000008023983b9 in abort () from /lib/libc.so.7
#3  0x00000000004392b5 in assertion_failed (file=0x66e750 "resolver.c", line=4242, type=9249680, cond=0x67027b "((fctx->finds).head == ((void *)0))") at main.c:252
#4  0x00000000006087ba in isc_assertion_failed (file=<value optimized out>, line=<value optimized out>, type=<value optimized out>, cond=<value optimized out>) at assertions.c:48
#5  0x000000000055dabe in fctx_unlink (fctx=0x18915) at resolver.c:4242
#6  0x000000000056e49d in maybe_destroy () at resolver.c:5275
#7  0x000000000056d4a4 in validated (task=<value optimized out>, event=0x84417eb30) at resolver.c:5360
#8  0x0000000000624c6f in run (queuep=<value optimized out>) at task.c:1128
#9  0x00000008020aec06 in pthread_create () from /lib/libthr.so.3
#10 0x0000000000000000 in ?? ()