[9.20] fix: dev: Disassociate the SSL object from the cached SSL_SESSION

When the SSL object was destroyed, it would invalidate all SSL_SESSION
objects including the cached, but not yet used, TLS session objects.

Properly disassociate the SSL object from the SSL_SESSION before we
store it in the TLS session cache, so we can later destroy it without
invalidating the cached TLS sessions.

Closes #4834

Backport of MR !9271

Merge branch 'backport-4834-detach-SSL-from-cached-SSL_SESSION-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!9274

lib/isc/tls.c

@@ -1657,6 +1657,8 @@ isc_tlsctx_client_session_cache_keep(isc_tlsctx_client_session_cache_t *cache,
 		return;
 	}
 
+	SSL_set_session(tls, NULL);
+
 	isc_mutex_lock(&cache->lock);
 
 	name_len = strlen(remote_peer_name);