robustness improvements

* handling of stop = 0 when calculating sizes (avoid unsigned underflow)
* make sure groupLength() is never zero (to avoid div/0)
* gapmaps: check for "(gapSize > 0)" added.

not sure if all the checks are 100% needed, but they will improve robustness in corner cases.

wled00/FX_fcn.cpp

@@ -1050,7 +1050,7 @@ uint32_t Segment::getPixelColor(int i)
   i += start;
   /* offset/phase */
   i += offset;
-  if (i >= stop) i -= length();
+  if ((i >= stop) && (stop>0)) i -= length(); // WLEDMM avoid negative index (stop = 0 is a possible value)
   return strip.getPixelColor(i);
 }