assertion failure in dns_xfrin_unref() on shutdown

The following assertion failure happened on shutdown in stress system test (see job#3288239):

bin/tests/system/stress/ns3/named.run:

31-Mar-2023 12:09:16.966 xfrin.c:753: INSIST(__v > 0) failed
31-Mar-2023 12:09:16.970 /builds/isc-projects/bind9/lib/isc/.libs/libisc-9.19.12-dev.so(isc_backtrace+0x28) [0x7fa64fe352e7]
31-Mar-2023 12:09:16.970 /builds/isc-projects/bind9/lib/isc/.libs/libisc-9.19.12-dev.so(isc_backtrace_log+0x42) [0x7fa64fe353cd]
31-Mar-2023 12:09:16.970 /builds/isc-projects/bind9/bin/named/.libs/named() [0x427e1d]
31-Mar-2023 12:09:16.970 /builds/isc-projects/bind9/lib/isc/.libs/libisc-9.19.12-dev.so(isc_assertion_failed+0x3e) [0x7fa64fe34d65]
31-Mar-2023 12:09:16.970 /builds/isc-projects/bind9/lib/dns/.libs/libdns-9.19.12-dev.so(dns_xfrin_unref+0x94) [0x7fa64fd27c7e]
31-Mar-2023 12:09:16.970 /builds/isc-projects/bind9/lib/dns/.libs/libdns-9.19.12-dev.so(dns_xfrin_shutdown+0x46) [0x7fa64fd28b51]

Observed on main, unsure if other branches are affected as well.