Fix: potential panic in putMsgToCache (#2634)

When the upstream DNS server returns a message that contains no
questions (i.e. QDCOUNT == 0), `putMsgToCache` will trigger an
out-of-range panic.

Issue: #2524
Comment: https://github.com/Dreamacro/clash/issues/2524#issuecomment-1477477601

dns/util.go

@@ -16,9 +16,9 @@ import (
 	D "github.com/miekg/dns"
 )
 
-func putMsgToCache(c *cache.LruCache, key string, msg *D.Msg) {
+func putMsgToCache(c *cache.LruCache, key string, q D.Question, msg *D.Msg) {
 	// skip dns cache for acme challenge
-	if q := msg.Question[0]; q.Qtype == D.TypeTXT && strings.HasPrefix(q.Name, "_acme-challenge") {
+	if q.Qtype == D.TypeTXT && strings.HasPrefix(q.Name, "_acme-challenge.") {
 		log.Debugln("[DNS] dns cache ignored because of acme challenge for: %s", q.Name)
 		return
 	}