VP80 fourcc are writed for all contexts (without ctx->codec_tag)
how to reproduce the issue:
1) Get any vp9 video (for example  http://base-n.de/webm/out9.webm)
2) ffmpeg  -i out9.webm -vcodec copy out9.ivf
3) out9.ivf have VP80 fourcc at ivf header
The proposed fix solves this issue
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>