如果 asset 中没有音频轨道
却调用了如下方法给 writer 添加了一个 input 就会导致报错:
-[AVAssetWriterInput appendSampleBuffer:] Must start a session (using -[AVAssetWriter startSessionAtSourceTime:) before appending sample buffers’
AVAssetWriterInput *assetWriterAudioInput = [AVAssetWriterInput assetWriterInputWithMediaType:AVMediaTypeAudio outputSettings:[self writerOutputAudioSettings]];
// Add the input to the writer if possible.
if ([assetWriter canAddInput:assetWriterAudioInput]) {
[assetWriter addInput:assetWriterAudioInput];
}