当使用AVAssetWriter获取原始字节时，需要确保数据的正确性和完整性。这个问题可能是由于在写入时丢失了某些数据或指针错误引起的。因此，您可以在开始写入前打印一些调试信息以查找错误。

以下是示例代码，用于演示如何使用AVAssetWriter以安全的方式获取原始字节数据：

// 初始化写入器
AVAssetWriter *writer = [[AVAssetWriter alloc] initWithURL:fileURL fileType:AVFileTypeMPEG4 error:&error];

// 设置输出格式
NSDictionary *outputSettings = @{AVVideoCodecKey: AVVideoCodecTypeH264,
                                 AVVideoWidthKey: @(1920),
                                 AVVideoHeightKey: @(1080)};
AVAssetWriterInput *writerInput = [[AVAssetWriterInput alloc] initWithMediaType:AVMediaTypeVideo outputSettings:outputSettings];
[writer addInput:writerInput];

// 初始化编码器
NSDictionary *sourcePixelBufferAttributesDictionary = @{(NSString *)kCVPixelBufferPixelFormatTypeKey: @(kCVPixelFormatType_32BGRA)};
AVAssetWriterInputPixelBufferAdaptor *adaptor = [[AVAssetWriterInputPixelBufferAdaptor alloc] initWithAssetWriterInput:writerInput sourcePixelBufferAttributes:sourcePixelBufferAttributesDictionary];

// 开始写入
[writer startWriting];
[writer startSessionAtSourceTime:kCMTimeZero];

// 获取原始字节
UInt32 dataLength = 0;
void *outputBytes = nil;
CGImageRef image = [UIImage imageNamed:@"test.png"].CGImage;
CGFloat width = CGImageGetWidth(image);
CGFloat height = CGImageGetHeight(image);
CFDataRef pixelData = CGDataProviderCopyData(CGImageGetDataProvider(image));
UInt8* pixelBytes = (UInt8 *)CFDataGetBytePtr(pixelData);

// 将数据写入文件
CMSampleBufferRef sampleBuffer = NULL;
CVPixelBufferRef buffer = NULL;
CMFormatDescriptionRef format = NULL;

CMVideoFormatDescriptionCreateForImageBuffer(NULL, buffer, &format);
CMSampleTimingInfo timing = {kCMTimeInvalid, kCMTimeZero, kCMTimeInvalid};

CMBlockBufferCreateWithMemoryBlock(kCFAllocatorDefault,
                                   pixelBytes, dataLength,
                                   kCFAllocatorNull,
                                   NULL, 0, dataLength,
                                   kCMBlockBufferAlwaysCopyDataFlag,
                                   &buffer);
CMSampleBufferCreateForImageBuffer(kCFAllocatorDefault,
                                    buffer,
                                    true,
                                    NULL,
                                    NULL,
                                    format,