The decoder returns with AVFrame.format not correctly set for some h264
files (strangely only some). We have to access AVCodecContext.pix_fmt
instead. On newer libavcodec versions, it's the other way around: the
AVCodecContext.pix_fmt may be incorrectly set on pixel format changes,
and you are supposed to use AVFrame.format.
The same problem probably exists on older ffmpeg versions too.