plugin/rewrite: Fixes a nil pointer dereference panic in ResponseReverter.WriteMsg (#8519)

ResponseReverter.WriteMsg calls res1.Copy() without checking res1 for
nil, so any plugin further down the chain that writes a nil response
(for example a handler returning (dns.RcodeSuccess, nil) after
w.WriteMsg(nil)) panics here.

Return an error instead, mirroring the nil guard recently added to
plugin/cache's ResponseWriter.WriteMsg (#8512).

Signed-off-by: zjncs <18910855655@163.com>

plugin/rewrite/reverter.go

@@ -1,6 +1,8 @@
 package rewrite
 
 import (
+	"fmt"
+
 	"github.com/miekg/dns"
 )
 
@@ -84,6 +86,9 @@ func NewResponseReverter(w dns.ResponseWriter, r *dns.Msg, policy RevertPolicy)
 
 // WriteMsg records the status code and calls the underlying ResponseWriter's WriteMsg method.
 func (r *ResponseReverter) WriteMsg(res1 *dns.Msg) error {
+	if res1 == nil {
+		return fmt.Errorf("rewrite: response message is nil")
+	}
 	// Deep copy 'res' as to not (e.g). rewrite a message that's also stored in the cache.
 	res := res1.Copy()