robustness: prevent negative transitions

fix for a corner case that might lead to unexpected math overflow

wled00/led.cpp

@@ -194,7 +194,7 @@ void handleTransitions()
   if (transitionActive && transitionDelayTemp > 0)
   {
     float tper = (millis() - transitionStartTime)/(float)transitionDelayTemp;
-    if (tper >= 1.0f)
+    if ((tper >= 1.0f) || (tper < 0.0f)) // WLEDMM also catch "<0" case, that may happen when millis() rolls over
     {
       strip.setTransitionMode(false);
       // restore (global) transition time if not called from UDP notifier or single/temporary transition from JSON (also playlist)
@@ -207,7 +207,7 @@ void handleTransitions()
     }
     if (tper - tperLast < 0.004f) return;
     tperLast = tper;
-    briT = briOld + ((bri - briOld) * tper);
+    briT = briOld + ((int(bri) - int(briOld)) * tper); // WLEDMM avoid undefined behaviour when briOld < bri
 
     applyBri();
   }