[InterNetNews/inn] f5f297: Fix gcc warning for arithmetic on pointers

[Julien ÉLIE]

  Branch: refs/heads/2.6
  Home:   https://github.com/InterNetNews/inn
  Commit: f5f29722bd6c199950e0ecfa61a17e175ff13036
      https://github.com/InterNetNews/inn/commit/f5f29722bd6c199950e0ecfa61a17e175ff13036
  Author: Julien ÉLIE <Julien-Elie at users.noreply.github.com>
  Date:   2021-09-18 (Sat, 18 Sep 2021)

  Changed paths:
    M storage/buffindexed/buffindexed.c

  Log Message:
  -----------
  Fix gcc warning for arithmetic on pointers

Arithmetic on a pointer to void is a GNU extension [-Wpointer-arith].
Cast it to char.