[InterNetNews/inn] cf6587: ctlinnd: Fix a format-truncation gcc warning

[Julien ÉLIE]

  Branch: refs/heads/main
  Home:   https://github.com/InterNetNews/inn
  Commit: cf6587829dcf5d5ea8958ba8400122be1acf6a97
      https://github.com/InterNetNews/inn/commit/cf6587829dcf5d5ea8958ba8400122be1acf6a97
  Author: Julien ÉLIE <Julien-Elie at users.noreply.github.com>
  Date:   2023-01-08 (Sun, 08 Jan 2023)

  Changed paths:
    M innd/cc.c
    M innd/icd.c

  Log Message:
  -----------
  ctlinnd: Fix a format-truncation gcc warning

Use xasprintf() instead of snprintf().

Keep the check on the length of newsgroup names, but instead of taking the
size of a buffer, take the maximum length of an NNTP argument.