Memory leak when port goes out of scope on Unix

[reconbot]

• SerialPort Version: 6.2.0
• NodeJS Version: all
• Operating System and Hardware Platform: unix/linux

Summary of Problem

#1572 fixes a memory leak in unix when the poller doesn't free it's callback. It's fixed by explicitly feeing the callback, but this wont work if the poller object goes out of scope.

[shodan8192]

Poller object can go out of scope only with whole SerialPort object - when that happen without prior call port.close() (what will free Poller) we have also port that is open without possibility to close or use it again. I'm not sure if there's need to handle this case. Even if the user code intentionally let SerialPort go out of scope (e.g after use it once), the leak will be insignificant.