mdns: Fix potential null dereference identified by fuzzer tests

components/mdns/mdns.c

@@ -2775,7 +2775,8 @@ static bool _mdns_question_matches(mdns_parsed_question_t * question, uint16_t t
         }
     } else if (service && (type == MDNS_TYPE_SRV || type == MDNS_TYPE_TXT)) {
         const char * name = _mdns_get_service_instance_name(service->service);
-        if (name && question->host && !strcasecmp(name, question->host)
+        if (name && question->host && question->service && question->proto && question->domain
+            && !strcasecmp(name, question->host)
             && !strcasecmp(service->service->service, question->service)
             && !strcasecmp(service->service->proto, question->proto)
             && !strcasecmp(MDNS_DEFAULT_DOMAIN, question->domain)) {
@@ -3237,7 +3238,7 @@ void mdns_parse_packet(mdns_rx_packet_t * packet)
                     } else if (service) { // only detect srv collision if service existed
                         col = _mdns_check_srv_collision(service->service, priority, weight, port, name->host, name->domain);
                     }
-                    if (col && (parsed_packet->probe || parsed_packet->authoritative)) {
+                    if (service && col && (parsed_packet->probe || parsed_packet->authoritative)) {
                         if (col > 0 || !port) {
                             do_not_reply = true;
                             if (_mdns_server->interfaces[packet->tcpip_if].pcbs[packet->ip_protocol].probe_running) {