delv crash when server responds with REFUSED

Summary

delv crashes when server responds with REFUSED (?)

BIND version used

• Affects v9.19: a695d141
• v9.18: 5a14d4b0 is not affected
• v9.16: d14a22b3 is not affected

Steps to reproduce

1. delv @ns2.dns-oarc.net. +dnssec _.sjc.dns-oarc.net DS

What is the current bug behavior?

$ delv @ns2.dns-oarc.net. +dnssec _.sjc.dns-oarc.net DS
;; chase DS servers resolving 'dns-oarc.net/DS/IN': 216.66.70.206#53
;; REFUSED unexpected RCODE resolving 'net/NS/IN': 2001:470:b2::206#53
;; REFUSED unexpected RCODE resolving 'net/NS/IN': 216.66.70.206#53
resolver.c:10612: REQUIRE(((nameservers) != ((void *)0) && ((const isc__magic_t *)(nameservers))->magic == ((('D') << 24 | ('N') << 16 | ('S') << 8 | ('R'))))) failed, back trace
/usr/lib/libisc-9.19.10-dev.so(+0x375c8)[0x7f28a2a375c8]
/usr/lib/libisc-9.19.10-dev.so(isc_assertion_failed+0x31)[0x7f28a2a374d9]
/usr/lib/libdns-9.19.10-dev.so(dns_resolver_createfetch+0x1a0)[0x7f28a27a770d]
/usr/lib/libdns-9.19.10-dev.so(+0x19d7f3)[0x7f28a279d7f3]
/usr/lib/libisc-9.19.10-dev.so(+0x6b194)[0x7f28a2a6b194]
/usr/lib/libisc-9.19.10-dev.so(+0x6a551)[0x7f28a2a6a551]
/usr/lib/libisc-9.19.10-dev.so(+0x489c6)[0x7f28a2a489c6]
/usr/lib/libuv.so.1(+0x141f9)[0x7f28a29aa1f9]
/usr/lib/libuv.so.1(uv_run+0xee)[0x7f28a29a3d16]
/usr/lib/libisc-9.19.10-dev.so(+0x525b4)[0x7f28a2a525b4]
/usr/lib/libisc-9.19.10-dev.so(+0x527d4)[0x7f28a2a527d4]
/usr/lib/libisc-9.19.10-dev.so(isc_loopmgr_run+0x17c)[0x7f28a2a53a47]
delv(main+0x1fb)[0x55b33a04f352]
/usr/lib/libc.so.6(+0x23290)[0x7f28a243c290]
/usr/lib/libc.so.6(__libc_start_main+0x8a)[0x7f28a243c34a]
delv(_start+0x25)[0x55b33a04a045]
Aborted (core dumped)

PCAP: delv.pcap