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.
Oh well, we are not calling an EventEmitter. So ignore my previous comment.
However, I'm not really a fan of depending on lodash for a simple once().
But feel free to do it as you wish.