[theora] ffmpeg2theora: Partial encode with 2-pass encoding results in crash

[ogg.k.ogg.k at googlemail.com]

> My command: ffmpeg2theora -s 0 -e 30 -V 512 --two-pass --optimize -x
> 640 -y 360 -A 64 -c 2 -H 44100 /Users/jason/Movies/Torchwood/S03E01.mkv

Fixed in SVN (partly, the crash is gone, but there's still some wonkiness,
which I'll look at now).
Sorry about that.