Fix startup panic when cascade data is missing

detector/detector.go

@@ -24,10 +24,15 @@ const (
 var classifier *pigo.Pigo
 
 func init() {
-	classifier, _ = pigo.NewPigo().Unpack(cascade)
+	if len(cascade) > 0 {
+		classifier, _ = pigo.NewPigo().Unpack(cascade)
+	}
 }
 
 func detectFaces(params *pigo.CascadeParams, angles ...float64) []pigo.Detection {
+	if classifier == nil {
+		return nil
+	}
 	// initialize angles if empty.
 	if len(angles) == 0 {
 		angles = []float64{0.0}