Undefined variables/array keys cause lots of PHP notices

Undefined variables/array keys cause lots of PHP notices on the settings page. That is, if you have strict error reporting enabled. This isn't a big problem, but here's a patch to fix them.

Also, one of the changes seems to be needed in order for the $can_audio_decode and $can_video_decode variables to be valid in each iteration of the foreach loop in ffmpeg_wrapper_can_decode().