Fixed an issue where video streams without FPS would raise exceptions

I used to get an exception when trying to fetch metadata information for an MP3 file with a single frame mjpeg video source used to display album art (cant share the original MP3 file due to copyrights).

The solution is a simple patch around parsing FPS which now ensures that a value is available and if not defaults to 0. Defaulting to null would be even nicer but that would break the API.

Included you'll find the patch plus a unittest to reproduce the issue and prove that its fixed