[InterNetNews/inn] a001a5: Fix gcc warning for arithmetic on pointers

[Julien ÉLIE]

  Branch: refs/heads/main
  Home:   https://github.com/InterNetNews/inn
  Commit: a001a5fb9612619f1c7a95f3a02be00fade18ab6
      https://github.com/InterNetNews/inn/commit/a001a5fb9612619f1c7a95f3a02be00fade18ab6
  Author: Julien ÉLIE <Julien-Elie at users.noreply.github.com>
  Date:   2021-08-12 (Thu, 12 Aug 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.