[InterNetNews/inn] b1e6e6: Fix potential overrun in concat()

[Richard Kettlewell]

  Branch: refs/heads/main
  Home:   https://github.com/InterNetNews/inn
  Commit: b1e6e683f6169a22f5d4aa698b02d7ae9daff833
      https://github.com/InterNetNews/inn/commit/b1e6e683f6169a22f5d4aa698b02d7ae9daff833
  Author: Richard Kettlewell <rjk at greenend.org.uk>
  Date:   2021-09-18 (Sat, 18 Sep 2021)

  Changed paths:
    M lib/concat.c

  Log Message:
  -----------
  Fix potential overrun in concat()

re #154

The inputs are mostly tiny so attempts to optimize the copying doesn't
seem likely to be rewarding.