analyzer: make http3/quic handling more reliable

analyzer/udp/internal/quic/payload_test.go

@@ -55,3 +55,14 @@ func TestExtractCryptoFrames_UnknownAfterCrypto(t *testing.T) {
 		t.Fatalf("frame0 = %+v, want offset=0 data=abc", frames[0])
 	}
 }
+
+func TestReadCryptoFrames_NonInitialHeader(t *testing.T) {
+	// Short header packet marker should be rejected as non-initial.
+	_, err := ReadCryptoFrames([]byte{0x40, 0x01, 0x02, 0x03, 0x04})
+	if err == nil {
+		t.Fatal("ReadCryptoFrames() error = nil, want non-nil")
+	}
+	if err.Error() != ErrNotInitialPacket.Error() {
+		t.Fatalf("ReadCryptoFrames() error = %v, want %v", err, ErrNotInitialPacket)
+	}
+}