Andrew Gregory authored Dec 16, 2017 and Allan McRae committed Jan 06, 2018

Frontends rely on an initialization call for setup between downloads.
Checking for intialization after checking for a completed download can
skip initialization in cases where files are small enough to be
downloaded all at once (FS#56408).  Relying on previous download size
can result in multiple initializations if there are multiple
non-transfer events prior to the download starting (fS#56468).

Introduce a new cb_initialized variable to the payload struct and use it
to ensure that the callback is initialized exactly once prior to any
actual events.

Fixes FS#56408, FS#56468

Signed-off-by: Andrew Gregory <andrew.gregory.8@gmail.com>
Signed-off-by: Allan McRae <allan@archlinux.org>