serveLiveLeds: Use variable buffer size

Allocate the serialization buffer size at the required length, rather
than always allocating the maximum size.

wled00/json.cpp

@@ -1152,7 +1152,7 @@ bool serveLiveLeds(AsyncWebServerRequest* request, uint32_t wsClient)
   }
 #endif
 
-  DynamicBuffer buffer(9 + (9*MAX_LIVE_LEDS) + 7 + 5 + 6 + 5 + 6 + 5 + 2);  
+  DynamicBuffer buffer(9 + (9*(1+(used/n))) + 7 + 5 + 6 + 5 + 6 + 5 + 2);  
   char* buf = buffer.data();      // assign buffer for oappnd() functions
   strncpy_P(buffer.data(), PSTR("{\"leds\":["), buffer.size());
   buf += 9; // sizeof(PSTR()) from last line