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.