'CFRelease' is unavailable: Core Foundation objects are automatically memory managed

@discussion
     The client is responsible for calling CFRelease on the returned CMSampleBuffer object when finished with it. This method will return NULL if there are no more sample buffers available for the receiver within the time range specified by its AVAssetReader's timeRange property, or if there is an error that prevents the AVAssetReader from reading more media data. When this method returns NULL, clients should check the value of the associated AVAssetReader's status property to determine why no more samples could be read.
     */