[theora] ffmpeg2theora: issue converting from a png image list

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

> (gdb) backtrace
> #0 0x083f644f in memcpy ()
> #1 0x083aed17 in planarCopy (c=0x8a5b930, src=0xbfe7dc20,
> srcStride=0xbfe7dbf0, srcSliceY=0,
> srcSliceH=720, dst=0x8a52cc0, dstStride=0xbfe7dc00) at
> /usr/include/bits/string3.h:52
> #2 0x083abcec in sws_scale (c=0x8a5b930, src=0x8a52bf0,

This isn't the one I fixed then, but this stack hints that using the latest
version of ffmpeg (which I was using and it works for me with your
data and command line) might fix it, as it looks to be a possible bug
in libswscale (part of ffmpeg).