Bugfix: millis rollover (fix for #3870)

millis()/1000 rollover after 18h was not handled, truncating to 16bits after division fixes it.

wled00/FX_fcn.cpp

@@ -455,9 +455,9 @@ CRGBPalette16 IRAM_ATTR &Segment::currentPalette(CRGBPalette16 &targetPalette, u
 // relies on WS2812FX::service() to call it for each frame
 void Segment::handleRandomPalette() {
   // is it time to generate a new palette?
-  if ((millis()/1000U) - _lastPaletteChange > randomPaletteChangeTime) {
+  if (((millis()/1000U) & 0xFFFF) - _lastPaletteChange > randomPaletteChangeTime) {
         _newRandomPalette = useHarmonicRandomPalette ? generateHarmonicRandomPalette(_randomPalette) : generateRandomPalette();
-        _lastPaletteChange = millis()/1000U;
+        _lastPaletteChange = millis()/1000U; //take lower 16bits
         _lastPaletteBlend = (uint16_t)(millis() & 0xFFFF)-512; // starts blending immediately
   }
 
@@ -466,7 +466,7 @@ void Segment::handleRandomPalette() {
     // assumes that 128 updates are sufficient to blend a palette, so shift by 7 (can be more, can be less)
     // in reality there need to be 255 blends to fully blend two entirely different palettes
     if ((millis() & 0xFFFF) - _lastPaletteBlend < strip.getTransition() >> 7) return; // not yet time to fade, delay the update
-    _lastPaletteBlend = millis();
+    _lastPaletteBlend = millis(); //take lower 16bits
   }
   nblendPaletteTowardPalette(_randomPalette, _newRandomPalette, 48);
 }