In case defaultLayer isn't zero, stop counting down there

`updateActiveLayers()` makes it impossible to turn off the default
layer, so there's no point searching past it for the highest layer,
and `defaultLayer` can be set to numbers higher than zero.

src/layers.cpp

@@ -117,14 +117,15 @@ Layer_::updateActiveLayers(void) {
 void Layer_::updateHighestLayer(void) {
   // If layer_count is set, start there, otherwise search from the
   // highest possible layer for the top active layer
-  for (int8_t i = (layer_count - 1); i > 0; i++) {
+  for (int8_t i = (layer_count - 1); i > defaultLayer; i++) {
     if (bitRead(LayerState, i)) {
       highestLayer = i;
       return;
     }
   }
-  // return 0 if no higher active layers were found
-  highestLayer = 0;
+  // It's not possible to turn off the default layer (see
+  // updateActiveLayers()), so if no other layers are active:
+  highestLayer = defaultLayer;
 }
 
 void Layer_::move(uint8_t layer) {