fuzz: fix rewrite crash by fixing fuzz/do.go (#3178)

Automatically submitted.

plugin/pkg/fuzz/do.go

@@ -13,15 +13,19 @@ import (
 // Do will fuzz p - used by gofuzz. See Makefile.fuzz for comments and context.
 func Do(p plugin.Handler, data []byte) int {
 	ctx := context.TODO()
-	ret := 1
 	r := new(dns.Msg)
 	if err := r.Unpack(data); err != nil {
-		ret = 0
+		return 0 // plugin will never be called when this happens.
+	}
+	// If the data unpack into a dns msg, but does not have a proper question section discard it.
+	// The server parts make sure this is true before calling the plugins; mimic this behavior.
+	if len(r.Question) == 0 {
+		return 0
 	}
 
 	if _, err := p.ServeDNS(ctx, &test.ResponseWriter{}, r); err != nil {
-		ret = 1
+		return 1
 	}
 
-	return ret
+	return 0
 }