AVCodecContext extradata should be an AVCDecoderConfigurationRecord
when bitstream format is avcc. Simply concatenating the NALUs output
by x264_encoder_headers does not form a standard
AVCDecoderConfigurationRecord. The following cmd generates broken
file before the patch:

ffmpeg -i foo.mp4 -c:v libx264 -x264-params annexb=0 bar.mp4
Signed-off-by: Zhao Zhili <zhilizhao@tencent.com>