ansi-filter truncates files with unexpected/invalid character encoding

[krobelus]

If I run kak < repro.txt it only reads 420 lines, even though the file has 4000 lines.

repro.txt

It looks like there is a race where both the new input and the `|%opt{ansi_filter}` want to update the buffer. Probably a bug in Kakoune?

Ok this just because the file contained invalid UTF-8.
Will try to work on a fix, or at least print a warning.