[dovecot/core] 6f380c: push-notification: Fix crash in OX driver's deinit

[GitHub]

  Branch: refs/heads/master
  Home:   https://github.com/dovecot/core
  Commit: 6f380c1e809a53ce2561577b9fa4df39695809d6
      https://github.com/dovecot/core/commit/6f380c1e809a53ce2561577b9fa4df39695809d6
  Author: Timo Sirainen <timo.sirainen at dovecot.fi>
  Date:   2017-06-05 (Mon, 05 Jun 2017)

  Changed paths:
    M src/plugins/push-notification/push-notification-driver-ox.c

  Log Message:
  -----------
  push-notification: Fix crash in OX driver's deinit

The http_client doesn't always exist.