don't crash when no presets are passed

cmd/root.go

@@ -150,7 +150,9 @@ func milkbucket(cmd *cobra.Command, args []string) {
 	}
 	defer m.destroy()
 
+	if len(args) > 0 {
 		m.loadPreset(false)
+	}
 
 	running := true
 	for running {

cmd/window.go

@@ -67,18 +67,22 @@ func (m *milkDropWindow) setupPresets(presets []string) {
  * preset.
  */
 func (m *milkDropWindow) nextPreset(smooth bool) {
+	if m.preset.Len() > 0 {
 		m.preset = m.preset.Next()
 		m.loadPreset(smooth)
 	}
+}
 
 /*
  * prevPreset sets the milkDropWindow's preset to the one before the current
  * preset.
  */
 func (m *milkDropWindow) prevPreset(smooth bool) {
+	if m.preset.Len() > 0 {
 		m.preset = m.preset.Prev()
 		m.loadPreset(smooth)
 	}
+}
 
 /*
  * newMilkDropWindow returns a new milkDropWindow with the given width and