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_fcn.cpp

@@ -729,7 +729,7 @@ uint32_t Segment::getPixelColor(int i)
   i += start;
   /* offset/phase */
   i += offset;
-  if (i >= stop) i -= length();
+  if ((i >= stop) && (stop>0)) i -= length(); // avoids negative pixel index (stop = 0 is a possible value)
   return strip.getPixelColor(i);
 }