heater: update averaged temp when heater is disabled

averaged temp must be updated with every reading even if heater is off,
otherwise it would drift away significantly when tip heater is being switched on/off

SolderingPen_ESP32S2/heater.cpp

@@ -104,6 +104,7 @@ void TipHeater::_runnerHndlr(){
     // skip cycle if heater is disabled or tip is missing
     if (!enabled || t > TEMP_NOTIP){
       _t.calibrated = t;
+      _t.avg = t;
       // set pwm to 0 if active
       if (ledc_get_duty(LEDC_MODE, _ledc_channel)){
         ledc_set_duty(LEDC_MODE, _ledc_channel, 0);