FFmpegFrameGrabber should throw an exception when attempting to access stream metadata before calling start()

As stated in the title, when using `FFmpegFrameGrabber`, if `start()` is not called beforehand, methods such as `getImageWidth()` will return `0`. This behavior may cause errors to propagate downstream and lead to issues that are more difficult to trace. I believe an `IllegalStateException` should be thrown immediately to clearly indicate that this operation is invalid.