temp runaway: proper cast to prevent any overflow.

Marlin/temperature.cpp

       {
         *timer = millis();
       } 
-      else if ( (millis() - *timer) > period_seconds*1000)
+      else if ( (millis() - *timer) > ((unsigned long) period_seconds) * 1000)
       {
         SERIAL_ERROR_START;
         SERIAL_ERRORLNPGM("Thermal Runaway, system stopped! Heater_ID: ");