fix for random crashes on changing presets (UI)

Segments are created/deleted on-the-fly; it seems that "local leds" buffers did not follow properly.
* revise segment copy/move constructors
* de-optimize use of local leds (as they need to be re-allocated when segment size changes)
* minor stability improvements

wled00/ntp.cpp

@@ -228,7 +228,7 @@ bool checkNTPResponse()
 {
   ntpUdp.flush();
   int cb = ntpUdp.parsePacket();
-  if (!cb) return false;
+  if (!cb) {ntpUdp.flush(); return false;}    // WLEDMM flush buffer
 
   uint32_t ntpPacketReceivedTime = millis();
   DEBUG_PRINT(F("NTP recv, l="));