Fix a memory leak in CvCapture_FFMPEG::close
Fix a memory leak in CvCapture_FFMPEG::close

FFmpeg now requires that frames allocated with avcodec_alloc_frame are
freed with avcodec_free_frame.

opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv