plugin/cache: don't cache or panic on responses with no question (#8467)

An upstream may return a NOERROR message with an empty question section
(some plugins emit this during prefetch). response.Typify classifies it
as NoError, so key falls through to m.Question[0] and panics, taking
down the server. Skip caching such a malformed response and log a
warning instead.

Fixes #6051

Signed-off-by: Saleh <root@lr0.org>

plugin/cache/cache_test.go

@@ -2008,3 +2008,16 @@ func TestCacheWriteMsgNilResponse(t *testing.T) {
 		t.Error("Expected error when passing nil response to WriteMsg, got nil")
 	}
 }
+
+func TestKeyEmptyQuestion(t *testing.T) {
+	// A response with an empty question section must be reported as
+	// non-cacheable instead of panicking on m.Question[0]. Some plugins can
+	// emit such a message (e.g. during prefetch). Regression test for #6051.
+	m := new(dns.Msg)
+	m.Response = true
+	m.Rcode = dns.RcodeSuccess
+
+	if ok, _ := key("example.org.", m, response.NoError, false, false); ok {
+		t.Fatal("expected a response with an empty question section to be non-cacheable")
+	}
+}