opening a non existent serial, throws an exception trying to close a non opened port!

Calling `.close()` on a closed port, raises an exception. This happens when you try to open a non existent port. This patch fixes.
I agre with you about dependancies. I'll try to use eventEmitter.once or I'll implement something short. Please note that I've closed this issue. I've opened another one more complete with other fixes. Thanks for your time.