BIND can crash with assertion in the situation when a `tls` entry is used multiple times to establish an outgoing connection

BIND can crash with assertion in the situation when a tls entry is used multiple times to establish an outgoing connection to other servers via TLS.

04-Dec-2023 13:38:33.746 tls.c:1187: REQUIRE(pstore != ((void *)0) && *pstore != ((void *)0)) failed
04-Dec-2023 13:38:33.746 /home/artem/projects/isc/open/bind9/lib/isc/.libs/libisc-9.19.19-dev.so(isc_backtrace_log+0x49) [0x7ffff7f4e6d7]
04-Dec-2023 13:38:33.746 /home/artem/projects/isc/open/bind9/bin/named/.libs/named() [0x42c0c6]
04-Dec-2023 13:38:33.746 /home/artem/projects/isc/open/bind9/lib/isc/.libs/libisc-9.19.19-dev.so(isc_assertion_failed+0x31) [0x7ffff7f4e052]
04-Dec-2023 13:38:33.746 /home/artem/projects/isc/open/bind9/lib/isc/.libs/libisc-9.19.19-dev.so(isc_tls_cert_store_free+0x42) [0x7ffff7f87ebd]
04-Dec-2023 13:38:33.746 /home/artem/projects/isc/open/bind9/lib/dns/.libs/libdns-9.19.19-dev.so(dns_transport_get_tlsctx+0x6f1) [0x7ffff7de49f4]
04-Dec-2023 13:38:33.746 /home/artem/projects/isc/open/bind9/lib/dns/.libs/libdns-9.19.19-dev.so(+0x696e6) [0x7ffff7c696e6]
04-Dec-2023 13:38:33.746 /home/artem/projects/isc/open/bind9/lib/dns/.libs/libdns-9.19.19-dev.so(dns_dispatch_connect+0xb6) [0x7ffff7c69d32]
04-Dec-2023 13:38:33.746 /home/artem/projects/isc/open/bind9/lib/dns/.libs/libdns-9.19.19-dev.so(+0x20acf2) [0x7ffff7e0acf2]
04-Dec-2023 13:38:33.746 /home/artem/projects/isc/open/bind9/lib/dns/.libs/libdns-9.19.19-dev.so(dns_xfrin_create+0x2c0) [0x7ffff7e09469]
04-Dec-2023 13:38:33.746 /home/artem/projects/isc/open/bind9/lib/dns/.libs/libdns-9.19.19-dev.so(+0x255a9b) [0x7ffff7e55a9b]
04-Dec-2023 13:38:33.746 /home/artem/projects/isc/open/bind9/lib/isc/.libs/libisc-9.19.19-dev.so(isc__async_cb+0x186) [0x7ffff7f4e500]
04-Dec-2023 13:38:33.746 /nix/store/lsp8kyfkyi35shk51alffb4vsll7030q-libuv-1.46.0/lib/libuv.so.1(+0x10543) [0x7ffff7730543]
04-Dec-2023 13:38:33.746 /nix/store/lsp8kyfkyi35shk51alffb4vsll7030q-libuv-1.46.0/lib/libuv.so.1(+0x238e5) [0x7ffff77438e5]
04-Dec-2023 13:38:33.746 /nix/store/lsp8kyfkyi35shk51alffb4vsll7030q-libuv-1.46.0/lib/libuv.so.1(uv_run+0xb0) [0x7ffff77311c0]
04-Dec-2023 13:38:33.746 /home/artem/projects/isc/open/bind9/lib/isc/.libs/libisc-9.19.19-dev.so(+0x571e0) [0x7ffff7f6c1e0]
04-Dec-2023 13:38:33.746 /home/artem/projects/isc/open/bind9/lib/isc/.libs/libisc-9.19.19-dev.so(+0x6e669) [0x7ffff7f83669]
04-Dec-2023 13:38:33.746 /home/artem/projects/isc/open/bind9/lib/isc/.libs/libisc-9.19.19-dev.so(+0x6e6ad) [0x7ffff7f836ad]
04-Dec-2023 13:38:33.746 /nix/store/qn3ggz5sf3hkjs2c797xf7nan3amdxmp-glibc-2.38-27/lib/libc.so.6(+0x8b084) [0x7ffff6e62084]
04-Dec-2023 13:38:33.746 /nix/store/qn3ggz5sf3hkjs2c797xf7nan3amdxmp-glibc-2.38-27/lib/libc.so.6(+0x10d60c) [0x7ffff6ee460c]
04-Dec-2023 13:38:33.746 exiting (due to assertion failure)

In particular, the problem reveals itself when multiple threads are trying to initialise a transport-specific TLS context and associated data from the context of multiple threads, like in the following situation:

tls tls-v1.3 {
	protocols { TLSv1.3; };
	prefer-server-ciphers yes;
};

zone "example-1" {
	type secondary;
	primaries port 22168 { 10.53.0.1 tls tls-v1.3; };
	file "example-1.db";
	allow-transfer { any; };
};

zone "example-2" {
	type secondary;
	primaries port 22169 { 10.53.0.1 tls tls-v1.3; };
	file "example-2.db";
	allow-transfer { any; };
};

zone "example-3" {
	type secondary;
	primaries port 22170 { 10.53.0.1 tls tls-v1.3; };
	file "example-3.db";
	allow-transfer { any; };
};

The error handling code is not correct for this case, as in some cases, freeing a TLS certificate store is not required. In this particular case it can be NULL.

The problem does not reveal itself on each run.