Init next_auto_fan_check_ms to zero

Marlin/temperature.cpp

 #endif
 
 #if HAS_AUTO_FAN
-  millis_t Temperature::next_auto_fan_check_ms;
+  millis_t Temperature::next_auto_fan_check_ms = 0;
 #endif
 
 unsigned char Temperature::soft_pwm[HOTENDS];