Before adding uvlinesize check, I was seeing failures decoding
some video with ffmpeg compiled with --enable-gray and using AV_CODEC_FLAG_GRAY.

[mpeg2video @ 0x7fa193818c00] get_buffer() failed (stride changed: linesize=1280/1280 uvlinesize=0/640)
[mpeg2video @ 0x7fa193818c00] get_buffer() failed (stride changed: linesize=1280/1280 uvlinesize=0/640)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>