inactive segments robustness improvement

* Avoid uint16 underflow in width() and height(): stop > start is possible, and means "inactive segment".

Without these checks, it was possible that width() and height() produce VERY large values due to underflow.

wled00/FX_2Dfcn.cpp

@@ -191,6 +191,7 @@ uint32_t WS2812FX::getPixelColorXY(uint16_t x, uint16_t y) {
 uint16_t /*IRAM_ATTR*/ Segment::XY(uint16_t x, uint16_t y) {
   uint16_t width  = virtualWidth();   // segment width in logical pixels
   uint16_t height = virtualHeight();  // segment height in logical pixels
+  if ((width == 0) || (height == 0)) return 0; // softhack007 avoid div/0
   return (x%width) + (y%height) * width;
 }