[dovecot/core] f01fe9: lib: istream-multiplex - Minor optimization

[GitHub]

  Branch: refs/heads/master
  Home:   https://github.com/dovecot/core
  Commit: f01fe9df4240c5b15d0b1c4b485a38f7c5736781
      https://github.com/dovecot/core/commit/f01fe9df4240c5b15d0b1c4b485a38f7c5736781
  Author: Timo Sirainen <timo.sirainen at dovecot.fi>
  Date:   2017-10-10 (Tue, 10 Oct 2017)

  Changed paths:
    M src/lib/istream-multiplex.c

  Log Message:
  -----------
  lib: istream-multiplex - Minor optimization

There's no need to find channel when we already know it. This also helps
static analyzers to understand that req_channel can't be NULL.