adnsresfilter: Fix addrtextbuf buffer size

adnsresfilter: Fix addrtextbuf buffer size

We can write 18 bytes (including the trailing nul) to this.  This is
not actually a problem in real compiled code because: cbyte is
generally the next thing; cbytes's alignment means that there are a
further two bytes of padding; and we only write the next two bytes
(']' and a nul) in a situation where we are done with cbyte anyway.

But it should be fixed.

Reported-by: Ron Henderson
Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>