Fix some minor bugs

Hi Daniel, 

A guy asked for some help with the media_vlc_player plugin in the Opensesame forum this week. I had a look on the error message he posted and found two things that caused an error. After that his experiment worked, however, I don't know whether I broke something important by changing the code. Maybe you can have a look on it? If it's fine, it might be worth it to update the plugin, even though there is a newer (and better) video plugin available. At least that's what Sebastiaan suggested. What do you think about it?

Best, 
Eduard